Weather: 'Southerly flow' to plummet temperatures as low as 3C

Parts of the South Island are in for a chilly night on Wednesday as a "southerly flow" passes through.

MetService says most places in the South Island will drop to single-digit temperatures.

Overnight lows in parts of Southland and inland Canterbury could drop as low as 3C overnight, according to WeatherWatch.

"Tonight [Wednesday] temperatures drop further as the southerly flow is replaced by high pressure - locking in the cold air," WeatherWatch said in a statement.

"Coastal areas won't be so cool tonight, and by Thursday and Friday warmer airflows return."

In the North Island, a severe thunderstorm watch has been issued for the Bay of Plenty, Rotorua, Taupō, Gisborne, and Hawke's Bay regions.

"This watch is valid from 1pm to 7pm today [Wednesday]," MetService said on Twitter.

"Local downpours of 25-40mm/h [are] possible."