"At first I couldn't get a grip and I ended up using my chin to drag myself up," Mr Williams told 7 News.
"I was out on a boat with the boys and one of them said it would be pretty funny if I surfed a whale so I just did it," he explained.
Footage shows Mr Williams on top of the rotting whale carcass while his friends on the boat call out to him in a panic to tell him that there were sharks in the water around the dead animal.
He said that he was aware of the sharks, but didn't realise how dangerous his actions were until after he had climbed up.
"He [the shark] was too busy chomping on the whale so it wasn't too bad," he said.
Despite this, Mr Williams said he would not do it again if given the chance.
"I've done it, I don't need to do it again," he said.
"It was definitely a stupid act but I didn't mean any disrespect to anyone. Mum thinks I'm an idiot, and Dad isn't too proud either."
Maritime authorities have said that Mr Williams won't be fined as the whale was already dead and he didn't remove any part of the animal.
On Sunday fisheries officers were monitoring the whale to ensure that it was not interfered with again, and the carcass drifted towards Scarborough, just off the coast of Perth.
Mr Williams had earlier told Daily Mail Australia that he had climbed on top of the whale because the animal looked like it was in trouble.
"Basically the whale looked in distress and I tried to help it. But clearly I was too late [sic],' he said.
The whale has reportedly been in the ocean for several weeks, floating between Rottnest and Fremantle off the Western Australia coast.
Western Australia's Surf Life Saving warned that several tiger sharks and at least one white shark were in the water surrounding the 12-metre-long whale.
Twitter users quickly saw the funny side of the situation, with one commenting "Only in Australia would a guy be sitting on a carcass in the ocean whilst sharks are feeding on it..."
Another said, "If I came across a floating whale I'd probably have a sit on it too. There's a box to tick."
The whale is reportedly the responsibility of the Fremantle Port Authority, reported The West Australian.
The animals carcass is not anticipated to come ashore for some time, said a spokeswoman from the Department of Parks and Wildlife spokeswoman.
"The way the winds are blowing it will probably stay in that ocean space for some time," she said.
Other users questioned the legality of the man's actions, and with a spokesman from the Department of Fisheries labeling his behaviour as irresponsible.
"If sharks were feeding on that whale carcass when he swam over then that type of behaviour is highly risky," Regional manager Tony Cappelluti told The West Australian.
"It potentially could have had some critical consequences, not only for the person but also for the witnesses and other people that would have had to assist."
"Ninety-nine per cent of the population would see that behaviour as irresponsible."
Mr Cappelluti said it was extremely risky for the man to enter the water when he could see sharks circling the body, as the had likely been attracted to the whale carcass from a long way away.
"It is very risky to enter the water around that type of large food source... Irrespective of some type of adrenaline rush or whatever you're trying to get out of performing that type of act, it's highly risky," he said.
A Reddit user commented, "Some of you West Australians are crazy ba****ds."
This comes just a day after photos were released showing a man being trailed by a three-metre Great White shark while surfing.
Andy Johnston was out with a small group of surfers at West Beach near Esperance, south-east of Perth, on September 21 when people ashore spotted the dark shadow looming.
"It seemed just curious and I didn't want to give it a reason to chase me so I tried to behave casually and keep an eye on it," Andy Johnston said.
"But I didn't know it came up quite so close as I was paddling into the wave to come into the beach with the other lads."
Next trending article: Jetski stunt ends in disaster for Bay of Plenty man
