On average, the water at Tauranga had been 0.25C chillier than normal in December.
When asked if beach-goers would notice such a small difference, Ms Murray said that a person who swam regularly could well notice it.
"Someone who's in the water over the whole month, they will feel it," she said.
Tauranga's harbour water was measured yesterday at 18.4C, and was forecast to remain at a similar temperature for the next week.
A Niwa climate scientist earlier said that this year was the hottest-ever for Tauranga and Te Puke, with the summer also forecast to be warmer than usual.
The scientist, Brett Mullan, said air temperatures at Tauranga Airport and Te Puke were on average about 1C warmer than normal.