Llewellyn will suit up for his 12th Sri Chinmoy National Capital Swim and is the oldest to swim Lake Burley Griffin having celebrated his 83rd birthday on Thursday.

The former triathlete says swimming the distance at his age doesn't make the feat any more special - he laughs he is "really a bit of a whimp" because he wears a wetsuit.

Llewellyn was beginning to wonder whether he'd do it again this year after an injury limited him to "a couple of legs" in last year's relay event.

But he couldn't stay away, because swimming keeps him "out of mischief" and he "should probably win" the 80 to 85 age group because there probably won't be too many to compete with.

"I think that you've got to be a bit dense," Llewellyn joked.

"Dense is not good for flotation but you've got to be a bit dense to keep doing this long distance swimming. I just have a go.

"I haven't done the triathlons for a year because of injuries. I'm gradually just winding down a bit but I guess swimming's not too bad on me."

Llewellyn has always felt comfortable in the water - whether he is doing a long distance swim or spear fishing and scuba diving in New Guinea.

He loves the weightlessness he feels under water and says there is always something to be discovered.

"I've got lots of things that stand out, especially from the diving," Llewellyn said.

"I used to go on diving holidays on World War II wrecks and stuff like that which I enjoyed very much.

"I've done most of the Sri Chinmoy 5km swims, and when they started this long one I always wanted to do it so I just got stuck into that."

The event gets begins at the Molonglo River and winds up on the other side of the Kings Avenue bridge.