Chilly nights to last the week as South Island drops to sub-zero

A 'blocking ridge' has been identified as the cause of sub-zero temperatures and frosts across the South Island overnight.

Temperatures plummeted to as low as -4.3degC at Lake Pukaki in the Mackenzie Basin, while further south Ranfurly and Middlemarch slipped to a similarly chilly -3.6degC, and Christchurch sat at -1.8degC.

MetService meteorologist Tom Adams explained that a blocking ridge was what had caused temperatures to drop so low - but said it also meant clear skies in many regions during the day.

"A blocking ridge is one that extends right through the atmosphere and stays put for several days. They generally bring consecutive days of fine weather," he said.

"The long, hot start to 2013 was a much more extreme example. Sometimes murky cloud can get stuck underneath a ridge, especially in winter, and that's what we're seeing in Christchurch."

Mr Adams said that cloud burned off last night, which resulted in the frosts down south - and it looks as though the current blocking ridge will stay in place throughout the week.

MetService says New Zealanders in many regions can expect a stretch of about five rainless, blue-sky days for the first time since mid-May as the blocking ridge takes effect.

It bucks the trend of this winter, which MetService says has been a particularly wet one.

Temperatures are expected to rise on Wednesday, but warnings are in place for drivers to be aware of the presence of ice on the roads until cloud builds up at the end of the week.