And in the process, he has raised close to $35,000 for his best mate, Tye Angland.

With his body covered in stinger bites and fighting the current, water temperature and incredible fatigue, Robertson completed the 20 kilometre ocean swim in stunning style on Saturday.

Twelve months after coming up with the plan to take on the Rottnest Channel Swim, Robertson ticked it off his bucket list in a time of six hours, 38 minutes and 44 seconds.

"It's something you can't really prepare yourself for," Robertson said.

"I did a lot of training, a lot of work and prepared as well as I could of but the longest swim I done in the pool was 12 kilometres. With this one being 20 kilometres, against a current and everything, I expect to have swam 21, 22 kilometres potentially.

"In the first five to six kilometres I got bitten (by a stinger) 10 to 15 times. One across my forehead and that was the most painful. It subsides after about five to 10 minutes and now it's just itchy and inflamed with marks all over my body. It's ridiculous. The worst part was that you didn't know when they were coming because you can't see them. You feel the burn and sting and think 'not again'.

"It was tough. They say hypothermia is one of the main things that stop people from finishing it and even though the water temperature was 22 degrees, which is quite nice, by the 10-12 kilometre mark I started to get really cold. I got a thermostat with milo to try and keep me warm but it's more of a mental game as well."

The swim was made sweeter by the surprise appearance of Angland, who travelled on Robertson's support boat and greeted him at the finish line.

"It was unexpected...but it was pretty special," he said.

"To have him on the boat was quite beneficial, especially when it got pretty tough. I didn't have to look far for a reminder of the reasons I was doing it."

Robertson believes his accomplishment will hit him in the coming weeks and months.

"It's something that hasn't probably sunk in 100 per cent yet," he said.

"It won't be until the weeks and months that follow on that I feel the significance of what I competed.

"Just the build up to it. If I was going over to do it for my own experience, I would have done it quietly and the pressure wouldn't have been there.

"But when you're raising money and people are giving that to you, you have to put yourself out there and that causes pressure. You think what if something goes wrong, or that I can't do it."

Robertson was overwhelmed by the response to his swim, which has seen his fundraising reach $34,895, a figure that has exceeded his initial goal of $10,000.

It will go to Angland, who was left a quadriplegic after a race fall in Hong Kong in November 2018

"It's insane...quite humbling," he said.

Robertson thanked his support crew, headed by Luke Rutter, for their efforts. To donate, visit Rottnest Channel Swim for Tye page.

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