A record number of swimmers in nothing but red caps ran for the River Derwent. (Representational)

More than 1,900 people rose early on Saturday to take a chilly plunge on the Australian island state of Tasmania.

The annual swim takes place on the winter solstice, which marks the shortest day of the year in the southern hemisphere, and this year, a record number of swimmers in nothing but red caps ran for the River Derwent.

Joy Walter, a 91-year-old from Tasmania's northeast, was one of the 1,915 who dashed for the water at 7:42 a.m. (2142 GMT on Friday), when the air temperature was just 5 Celsius (41 Fahrenheit).

"I just thought I'd like to do that," Walter said. "I'll be 92 in 44 days, and so I thought I better do it while I still can."

Now in its seventh year, the nude swim is part of Dark Mofo, a three-week winter festival of music and art that features exhibitions, light and sound installations, and live performances across Tasmanian capital Hobart.