Constable Aaron Redaelli said he felt compelled to enter the lake about 2.15am when he realised the man was staying under the water longer and longer.
It was likely the man was in the chilly water for 15 minutes before the call was received, and another 15 minutes before he was located, "so if someone didn't go out there he might not have survived", Const Redaelli said.
"We could have been dragging a body out rather than a breathing human being, so it's worked out all right for him."
Const Redaelli was in Frankton when he and another constable received a call alerting them to what they originally thought were two swimmers struggling in the lake.
But police saw only one man bobbing in the water.
It was a "team effort to locate the guy, first and foremost", but it was his decision to enter the water after removing everything but his T-shirt and underwear.
"It wasn't just a spur-of-the-moment decision to swim out there, Const Redaelli said.
"I probably didn't take the cold into account as much as I should have - it sucks the energy out of you."
On the shoreline, Constables Jason Reid, Shona Campbell and Acting Sergeant Phil Hamlin shone torches on the pair, an action for which Constable Redaelli was grateful.
"At the end of the day, I want to go back to my family."
After struggling with the man in the water, Const Radaelli successfully pulled him back to shore, where both needed assistance.
With the air temperature at 0degC and the lake water at 11degC, the man was suffering from hypothermia. Const Redaelli was also affected by the cold.
"By the time I got back to shore, I was starting to lose [feeling in] my arms and legs because of the cold.
"When you get cold, you just sort of shut down."
An ambulance arrived as the pair were emerging from the water, and the man was taken to hospital for observation.
Const Radaelli, however, had a "slow, warm shower for 10 minutes" then went back on duty until 7am.
Sergeant Derek Ealson, of Queenstown, commended Const Redaelli's efforts and said the man was lucky to survive in the cold temperatures.
"Without the swift actions of Const Redaelli, this young man could have lost his life."
