IT HAS been a few years since Petria Thomas has done post-swim media interviews.

''At least I'm not puffing so much after this one,'' Thomas said after half an hour in the pool at the Australian Institute of Sport yesterday.

There were no medals on offer, and the only records to be broken were fundraising ones, as Thomas kicked off the Canberra 24-hour Mega Swim, which aims to raise $130,000 to support people with multiple sclerosis (MS).

Thomas, who retired after winning three gold medals at the 2004 Athens Olympics, doesn't spend much time in the pool these days, but was quick to sign up as the event's ambassador.

''It's a great cause and obviously an activity which is close to my heart as well,'' Thomas said.

Although swimming is a source of fond memories, it's no longer a regular feature of her life.

''I went for a swim earlier this week just to make sure I could still swim for half an hour,'' she said.

''I admire some of these people who are going to be here in the middle of the night swimming up and down, but it's for a great cause and that's what matters,'' Thomas said, acknowledging her Olympic teammate, Sarah Ryan, whose father had MS.

The three-time Olympian showed she still had some talent, managing 1.9 kilometres in her 30 minutes, which she rated as ''not very good considering I used to be able to swim 2km in about 20 or 21 minutes''.

But speed and lap numbers was not the focus of the event, which has 29 teams of up to 15 people swimming in shifts until midday today.

With music playing, and a barbecue firing outside to feed the hungry participants, the most competition is in the fundraising stakes, with fierce rivalries developing between some of the regular teams.