There will be relief for many people in the South Island after a hot and sticky night - but that heat is heading north.

Records for December temperatures were broken in Christchurch and Dunedin yesterday, including 36.1 degrees at Christchurch airport.

Weather expert Richard Green says it has cooled off in central and southern areas overnight, and there's a cooler wind change pushing through north Canterbury.

Gisborne, Napier, Hastings and possibly Masterton are going to feel the heat today, "maybe not to the same degree as the south yesterday...It's still going to be hot."

Green says Christmas Day will be fine in up to 90 percent of the country but New Year's will be more unsettled.