Eyewitness Hannah Murphy told CNN: "Walking towards the beach it looked to me like a huge wave coming from the back of the ocean about to swamp us all!"
"People were running from the streets to capture the unbelievable cloud formation."
The Belfast Telegraph reports that the Australian Bureau of Meteorology issued a severe thunderstorm warning for some areas in Sydney.
According to Wikipedia, an arcus cloud is a low, horizontal cloud formation. Roll clouds and shelf clouds are the two types of arcus clouds. A shelf cloud is usually associated with the leading edge of thunderstorm outflow; roll clouds are usually formed by outflows of cold air from sea breezes or cold fronts in the absence of thunderstorms.
18 weird natural phenomena around the world
Incredible 'tsunami cloud' rolls in over Sydney (video)
Dried-out treetops protrude from crystal water in an eerily beautiful liquid landscape - the result of an enormous earthquake more than 100 years ago. As Lake Candy sits 2,000m above sea level, the temperatures here remain icy: they rarely exceed 6C. This coldness has preserved the submerged trees. Great pine cones still remain on the trees underwater from 100 years ago: you can see into the great depths of the lake through the clear mountain water.
Abraham Lake on the North Saskatchewan River in Alberta, Canada, is home to a rare phenomenon where "ice bubbles" form just beneath the water's surface. These are formed by plants on the lake bed releasing methane gas, which freezes as it nears the surface. During the cold winter, these bubbles stack up, making beautiful patterns beneath the ice.
Where stable moist air flows over a range of mountains, a series of large-scale standing waves may form on the downwind side. Lenticular clouds sometimes form at the crests of these waves, and, because of their shape, they are often mistaken for UFOs. This photo shows a lenticular cloud at Katmai National Park, Alaska.
The Cueva de los Cristales, 300m under the ground in the remote area of Naica, Chihuahua, contains extraordinary giant selenite crystals, some of the largest ever found in the world. The longest so far measures 12m, 4m in diameter and weighs 55 tonnes. The cave was discovered by accident in 2000 by miners drilling for silver and lead. Temperatures in the cave reach 58C with 99 per cent humidity, so it remains relatively unexplored - but it's a source of fascination for scientists and geologists trying to unlock its secrets.
It's no surprise that photographers flock to capture the unique light and landscape of Bolivia's Salr de Uyuni. This 10,000 square km region, the result of transformations between several prehistoric lakes, is home to the world's largest supply of lithium reserves. The salt is more than 10m thick at the centre. In the wet season it is covered by a thin film of water that it is often referred to as the world's largest natural mirror.
Theories abound as to the cause of the strange circular patches of land known as 'fairy circles' which stretch for more than a thousand miles in the Namib desert. Everything from radioactive soil to UFOs and meteorites have been blamed, while local myth holds they are caused by the fiery breath of a dragon slumbering beneath the earth. However, in 2013, German professor Norbert Jurgens, who had studied the phenomenon for six years, claimed to solve the mystery by concluding that a species of sand termite is responsible.
These large spherical stones, scattered along the beach 40km south of Omaru, are formed from ancient sea floor sediments around 60 million years ago. Scientists believe four million years of crystalisation gradually formed the boulders into the pearl-like structures that they are today. Maori legend holds that the boulders are gourds washed ashore from the great voyaging canoe Araiteuru when it became shipwrecked. Each boulder weighs several tonnes and stands up to two metres high.
The mysterious Blue Hole of Belize is believed to be the world's largest sink hole, measuring more than 300 metres across and 125 metres deep. The hole originally formed as a limestone cave in the glacial period. Rising sea levels over millions of years meant the cavern was flooded and eventually collapsed, creating an incredible 'vertical cave'.
A circumhorizontal arc or fire rainbow is a rare optical phenomenon - an ice-halo formed by plate-shaped ice crystals in high-level cirrus clouds. They can be seen best when the sun is very high in the sky - higher than 58°. Apparently, you have to be at a very precise location and latitude so see one.
This beautiful phenomenon occurs when plant stems and roots are not quite frozen and full of moisture. As that water is transmitted up to the higher part of the stem, it freezes, creating layers of ice. Frost flowers are usually created on autumn or early winter mornings. This extrusion creates wonderful patterns which curl and fold into gorgeous frozen flower like shapes.
No - this image was not created using Photoshop. Goats really do climb the Argan trees in the village of Tamri, Morocco. They are in search of food, which is otherwise sparse in the region. Argan berries are rich in nutrition, but reaching the fruit requires the animals to perform an extraordinary balancing act that’s quite unexpected from a hoofed animal. Over time the local goats have become extraordinarily adept at tree-climbers.
A thunderhead grows vertically instead of horizontally, forming into dense towers that can reach 20,000 feet and giving the appearance of tidal waves. They have been known to grow as high as 75,000 feet. The presence of thunderheads means that severe weather conditions like heavy rains, hail, lightning and thunder are imminent. Someone ought to point that out to the paddlers in this picture...
Believed to have formed over 2,700 million years ago, Australia's famous Wave Rock was created by gradual erosion over many centuries. The "wave" is about 14 m (46 ft) high and around 110 m (360 ft) long.
Deep in the earth beneath Pamukkale lies a vast source of water heated by volcanic lava. The water dissolves pure white calcium, becomes saturated with it, and carries it to the earth's surface, where it bursts forth and runs down a steep hillside. Cooling in the open air, the calcium precipitates from the water, adheres to the soil, and forms white calcium "cascades" frozen in stone
There's a giant hole in the Kara-kum Desert in Turkmenistan - and it's been on fire for more than 40 years. Derweze or Darvaza, is known as the 'Door to Hell' - for obvious reasons. This huge crater of burning natural gas was accidently formed by geologists who were drilling there in 1971. Their activity caused the ground to collapse, leaving a 70m wide hole that was leaking gas. The team tried to burn it off in the hope that the fuel would be spent within a few days. But it's still burning as brightly as ever.
On Christmas Island, between October and December, tens of millions of red crabs begin a truly spectacular migration from the forest to the coast to breed and spawn. It has been estimated that 43.7million red adult crabs lived on the island at one point, although the accidental introduction of the yellow crazy ant, which are strong enough to overwhelm and kill the crabs, is believed to have reduced this population in recent years. But the migration is still one of the world's weirdest sights to behold...
Mammatus clouds (or 'breast' clouds - guess why they're called that?) are rare pouch-like circular lobes that form after a thunderstorm. Their appearance is most visible when the sun is low in the sky, when the pouches look as if they're 'framed' by the light. It's believed that they are formed when evaporation causes pockets of buoyancy, making the clouds puff downward instead of upwards. "They're kind of upside down convection," explains Daniel Breed from the National Center for Atmospheric Research, speaking in wired.com.
A rare but extremely impressive phenomena, volcanic lightning can occur during particularly violent eruptions. Apparently, it's the result of positively charged matter being thrown skyward, causing separated electrical charges take shape, triggering lightning bolts - nature's way of balancing charge distribution. This picture shows Iceland's Eyjafjallajokull volcano.
Bioluminescence means the production of light by a living organism. This picture shows the island of Vaadhoo in the Maldives, where the phenomena causes a 'Sea of Stars'. It's caused by a species of plankton emitting a bright blue light as their cell membranes respond to electrical signals.