Swimmers shared Sydney Harbour with seven deadly sharks

The Sand Tiger shark is often docile, as the Sydney bull sharks appeared to be - PA

Swimmers who swam in Sydney Harbour will be unnerved by the news that they were sharing the water with seven dangerous bull sharks.

A study has revealed that the seven male sharks were lurking silently beneath the water while tens of thousands of revellers cooled off in temperatures of 30C on Australia Day, the busiest day of the year.

The bull sharks, which have been known to attack swimmers in Sydney Harbour, were among 16 fitted with monitoring devices by government scientists in a bid to find out which parts of the waterway they like to congregate in.

Sharks were, however, found throughout the harbour, from the heads in the east to Balmoral in the north and Concord in the west. All of the tagged sharks measured more than 6.5ft long but there were no sightings on that day, said a report in the Telegraph.

One 9ft long bull shark covered 22 miles during the day, visiting several busy spots where hundreds of people were enjoying a dip.

According to government scientists, the news is proof that sharks and humans can co-exist safely in the harbour.

"One could argue that bull sharks may not be the voracious predator we once thought," Dr Amy Smoothey told the Sydney Daily Telegraph.

The sharks were fitted with tags after Paul de Gelder, a Navy diver, lost a leg and hand in a shark attack in 2009 during a training exercise. It was the first shark attack in the harbour for nine years.