Almost 2000 people have stripped off for a record-breaking midwinter dip in Hobart to mark the shortest day of the year.

And the oldest was aged 91.

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First-timer Joy Walter, from St Helens on Tasmania's east coast, was one of the 1915 hardy souls who took the chilly solstice plunge in the River Derwent on Saturday morning.

"I only got in halfway," she said, rugged up and back on dry land.

"It was very cold and I fell in a hole, so there was no option to keep going."

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In preparation, Ms Walter did a pretend solstice swim with her late husband several years ago but was determined to do the real thing.

"I thought, 'I can have a real one now'," she said.

"It was a bit confronting (getting naked), as was getting dressed afterwards. But it was lovely.

"Maybe I'll be back next year. (Now) I've got the cap to prove it."

'Keep Tassie wild'

An orange flare was fired at 7.42am - the sunrise signal for red-capped swimmers to drop their towels and streak for the water.

Screams, shouts and wails followed.

Some took a political message into the river, with 'Keep Tassie Wild' and 'No Cable Car' - referencing the proposed development up Hobart's lofty Mount Wellington - written on peoples' backs in red paint.

The air temperature was a nippy 5 edgrees, while the temperature in the water was in the low teens.

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Hobart residents Josh Vallelonga and Sam Bell were also first-time nude swimmers, and said they'd like to return next year.

"We psyched ourselves up, ducked our shoulders in, started screaming and ran out," Ms Bell said.

"I'd definitely do it again. I'm ready for a hot bath."

Another couple took the opportunity to get married amongst the bare crowd.

The swim, a symbolic act of purification, is in its seventh year after 230 participants began the tradition in 2013.

About 2200 people aged 18 and over registered this year, but drizzly weather probably kept some in bed.

The record breaks the 2018 mark of 1537 swimmers.