Melbourne's Kim Rea the oldest participant in 2023's The Rip winter swim at Port Phillip Bay

Six months ago, Kim Rea could not swim 50 metres without losing his breath.

But today he will be the oldest of 26 people who will swim The Rip — the 3.2-kilometre treacherous stretch of water between Point Nepean and Point Lonsdale in Port Phillip Bay.

The 73-year-old loved going for ocean swims until about his mid-50s when he swapped freestyle for bushwalking after his arthritis got worse.

A two-decade break from the water was enough to spark Mr Rea's curiosity, so he took the plunge and got back into the pool earlier this year.

"I was very surprised when I couldn't travel 50 metres without stopping to try and get my breath back," he said.

Determined to find his former strength, Mr Rea went to the pool three times a week and quickly regained his ability to comfortably complete a lap.

At the same time, he wondered about doing a swim across Port Phillip Bay.

He knew swim coach Grant Siedle, who is the director of Rip Swim, facilitated these journeys, but only groups of swimmers were allowed to participate due to safety, and Mr Rea did not have a team.

"At my age, I don't know anybody who would be interested and capable," he said.

"I just decided it was all too hard."

The rules changed earlier this year. For the first time, Mr Siedle would accommodate solo swimmers.

Ports Victoria made an arrangement with him to stop cargo ships from travelling through the bay for about an hour and a half while the swimmers made their journey.

"Initially, because we had shipping to deal with, we needed to keep people together," Mr Siedle said.

"But the shipping arrangement meant I could offer solo spots."

Mr Rea quickly signed up and started training.

"It was an incentive for me to train for something that would increase my resilience and to achieve something I've always wanted to do."

But then Mr Rea was confronted with something he had not thought of: the cold water.

In honour of Doug Mew, who first crossed the bay without a wetsuit in 1971, participants are required to do the same — in 11 degrees Celsius water.

"If I thought I was short of breath, the cold water takes it all away," Mr Rea said.

So he changed his training regime, opting for the similarly cold Brighton Baths.

He swam there without a wetsuit three times a week, some days enjoying the sunshine and others battling the wind or a cramping leg.

After three months of training, he is feeling strong and confident for today's event.

"There's nothing about the swim that will prevent me from successfully crossing — from a distance point of view," he said.

Mr Rea is one of 26 swimmers taking part in the swim.

Mr Siedle — who has been running summer and winter Port Phillip Bay crossings for nearly 10 years — said he believed Mr Rea was the oldest person to swim The Rip during winter.

And this winter swim is long-awaited.

Mr Siedle has had to postpone the event by a month due to windy conditions, but the weather is looking good for this weekend.

About 780 swimmers have crossed The Rip over the last decade, and Mr Siedle has been part of each of their journeys from the start.

"I know every single swimmer personally, and they've all done a qualifying swim. It's the best way of making sure everyone's safe," he told ABC Radio Melbourne.

"It's just a thrill seeing people cross this body because it's such an incredible area."

As for whether Mr Rea will complete another ocean journey like this, he said this one would be it.

"This is a once-in-a-lifetime opportunity for me," he said.

In the meantime, Mr Rea is thinking about how good it will feel when he reaches the finish line.

"Two of my sons will be acting as my ground crew," he said.

"They'll have a fresh towel waiting for me."