Bondi man Cyril Baldock has set the record for the oldest person to swim the English Channel.

The 70-year-old Bondi Surf Life Saving Club member completed the swim between England and France in 12 hours and 45 minutes on Wednesday (local time).

It was his second Channel swim, after first completing the test - considered the 'Everest of swimming' - in 1985, when he became the fifth Australian to do so.

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After finishing his swim overnight he posted on Twitter: "Support from home has been overwhelming - thanks to everyone for following our journey".

Mr Baldock told the ABC his dream to break the world record began when his mentor and coach, Des Renford, had a heart attack.

Described as "the king of the channel", Renford had wanted to be the oldest person to achieve the record.

"Des wanted to have a crack at that but obviously it was never going to be," Mr Baldock said.

"I told Des that when I got to that age I'd have a go at it. But little did I know over that 30-year period the oldest went up 10 years, so I had to wait a while."

Mr Baldock said he would not have done the swim again if it was not for the record.

"There's no way that I'd attempt the English Channel just to swim it a second time. I've done it, I was only the fifth Australian to do it back then, but to be the oldest it's an enormous challenge. It's something that very few people can ever achieve," he said.

However Mr Baldock's record may be short-lived, because 73-year-old Brisbane woman Irene Keel is also attempting the swim this week.

Cyril Baldock swims across the English Channel at Cap Gris Nez in northern France. ( AAP/Supplied )

Mr Baldock's training consisted of around 40 kilometres of swimming each week, including five pool sessions and a long open water swim on a Saturday.

The English Channel is 34km straight across but it is very rare for a swimmer to cover it in under 40km.

Mr Baldock's coach Vladimir Mravec specialises in preparing people for marathon swims like the English Channel.

This year, 12 of his squad members are attempting the feat.

So far seven have been successful, including Sydney's Stuart Johnson, who was attempting to be only the fourth person to swim three laps of the Channel continuously.

Johnson managed two-and-a-half laps but decided to retire after 30 hours in the water, tired and close to hypothermia.

Bondi Surf Life Saving Club member Cyril Baldock with his coach Vladimir Mravec ( Brendan King )

"My role as coach is take his momentum and make the momentum live," Mravec said.

It is this momentum that not only helped Mr Baldock be best prepared for the Channel swim but also in other areas of his life.

"It's helped my fitness, I've got five grandkids and I'm able to do things with them that a lot of 70-year-olds couldn't do," Mr Baldock said in the lead-up to the swim.

"Swimming is the ultimate sport for fitness because you're under pressure all the time, you're breathing all the time."

Mr Baldock has lost a number of friends to skin cancer and is raising money for the Melanoma Institute of Australia.