MetService has issued new heavy rain watches, leaving most of New Zealand preparing for the possibility of dense downpours.

Watches are in place for Northland, Auckland, and the Coromandel, with periods of heavy rain potentially reaching warning criteria or exceeding, and the possibility of thunderstorms.

Waikato and Waitomo (including Taumarunui), Taupō, Tongariro National Park and the Kaimanawa Mountains all already have heavy rain watches in place until 5am Saturday, as does the Bay of Plenty (including Rotorua), ending at 8am.

A heavy warning has been issued for Mount Taranaki until 3am Saturday, and in Nelson until 7pm, with a further 70 to 110 mm of rain expected on top of what has already fallen in these areas and thunderstorms possible.

Until 6am Saturday, a heavy rain warning is also in place for the ranges of Westland (south of Otira) with a further 90mmto 120mm of rain on top of what has already fallen, peaking in the afternoon.

Another burst of heavy rain is expected overnight Saturday and early Sunday morning.

MetService warns that heavy rain may cause streams and rivers to rise rapidly. Surface flooding and slips are also possible, and driving conditions may be dangerous.

A heavy rain watch is in place until 7am Saturday for Buller, and 8pm Friday in the Richmond and Bryant ranges (including the Rai Valley), with periods of heavy rain which may reach warning criteria possible, and possible thunderstorms in Buller.

In Fiordland (north of Doubtful Sound), a 24-hour heavy rain watch is in place from 4am Friday with periods of heavy rain which may reach warning criteria and thunderstorms possible.

Another burst of heavy rain is expected in Fiordland on Saturday night and early Sunday morning.