20-degree weather on way as sub-tropical winds kick in

New Zealand's cold snap is about to be defeated as warmer weather sets in.

High pressure to our east will encourage a sub-tropical northerly flow, bringing temperatures that will be several degrees warmer than average.

NIWA says many places will experience their warmest temperatures so far this spring.

"To give you perspective, parts of the South Island have overnight lows this week that are very similar to the maximum daytime temperatures they had last week during the cold, wintry, southerlies!" WeatherWatch says.

"Daytime highs are well up with a number of places expected to make it to the 20C mark this week, maybe even the mid-20s for some spots in the east or inland."

At this stage, Wednesday looks like it will be the warmest day nationwide.

However after this comes a cool-down later in the week, with the southern half of the South Island to be worst affected.

"A low is also forming in the Tasman Sea and will likely bring a late-week burst of rain, showers and another South Island temperature drop," WeatherWatch says.

"With a damp, cold, wintry change later this Thursday, peaking Friday and easing Saturday some farmers in the lower South Island in particular may have tough wind chills for newborn livestock."

High pressure returns this upcoming weekend before the next low out of the Tasman Sea hits this time next week.