A group of Scion workers have braved the cold waters of Tikitapu (Blue Lake) for the 21st Scion Polar Plunge.
About 22 people took the plunge into the lake today, with the water sitting at 12C.
Scion worker Dean Meason said it was a tradition and was "fun in a crazy, silly kind of way".
He had taken part six times and said the weather felt tropically warm compared to previous years.
It was Michelle Easton's second time doing the plunge. She said "it makes you feel alive, it's fun".
Intern Pierre Belle from Brazil gave it a go for the first time, and said he was a bit anxious beforehand.
Mr Belle said he decided to do it to have fun and be a part of the tradition.
He said when he jumped in he got a rush, and then started to be able to tolerate it.
It was not the sort of thing they usually did in Brazil, and even the colder waters in Brazil were warmer than the lake, he said.
Toni Withers, said she had not done it for a few years and felt guilty, so thought she better do it this year.
"Once you dive in it really hits you."
Organiser and Scion scientist Garth Weinberg said he had participated in all of the plunges except for one.
He said he still got a shock every year, and that 12C was exactly average from previous readings.
The tradition started off as a challenge between two social groups at the crown-owned institute and took place on the shortest day each year, he said.
MetService meteorologist Emma Blades said Rotorua got up to 14.1C today, with plenty of cold weather still on the way, as the coldest months of July and August approach.