Jackson Vincent snapped these astounding photographs of the huge roo tensing its rippling muscles in Boodjidup Creek in Margaret River, Western Australia, on Tuesday.
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The 27-year-old couldn't believe his eyes when he clocked the mammoth 6ft 5in mammal standing in the water – and feared it could be preparing to attack beloved dog Dharma.
Jackson, from Freemantle, Perth, had been visiting his grandmother in Margaret River and saw the gargantuan animal on her property at about 11.30am.
He said: "I have been going to that creek since I was a little boy and there have always been kangaroos on the property.
"I was walking my dog and we saw this huge roo standing in the water. I have never seen a kangaroo standing in the water like that, we could just see his head at first, and he was one of the biggest I have ever seen.
"As I ran around him to take a picture he came closer and that was when I realised he was coming right at me and he was really big.
"I am pretty confident around animals but the moment when he started to come out of the water my heart definitely jumped and I decided to take a few steps back.
"He looked to be at least two metres (6ft 5in) tall or taller and must have weighed at least 100 kg (16st).
"He had a really big body and was taller than me. There are a lot of roos at my grandma's place but I have never seen one that muscular before – he was a big macho male.
"I decided to get Dharma out of there as we thought the kangaroo was going to lure him into the water to drown him."
Jackson's Facebook post with his images of the kangaroo's muscular physique quickly went viral, racking up thousands of likes and shares.
Online commenters were quick to point out the animal's impressive bodybuilder-like muscular shoulders, biceps and chest and threatening posture.
After dropping Kelpie Dharma back at his grandmother's house Jackson returned to the creek to see the roo was still there.
Jackson added: "His claws were really big – they were as long as my hand. That for me was even scarier than his muscles, they looked nasty.
"I have never seen a kangaroo be so defensive – it definitely looked like he was puffing his chest muscles up.
"I definitely wouldn't like to take him on in a boxing match."