Southern and central New Zealand is bracing for a massive storm, with torrential rain, thunderstorms and damaging gales in the next 24 hours.

The MetService says the country is about to be hit by classic Equinoctial gales hitting first at the bottom of the country early tomorrow morning and blasting both islands over the coming two days.

Virtually the entire South Island and much of the lower North Island is covered in more than a dozen wind and rain watches and warnings.

The worst of the weather will hit the bottom of the country with a whopping 200mm of torrential rain to fall in Fiordland from dawn tomorrow, and continue non-stop for the next 21 hours.

The fast-moving front would see powerful winds blowing up to 120km/h howl through Canterbury overnight on Tuesday and the lower North Island on Wednesday.

Heavy rain is also set to drench the West Coast with a staggering 240mm of rain expected to fall in the ranges and 110mm nearer the coast. Thunderstorms are also expected to roll over Fiordland.

The forecaster has issued a warning about the danger of flooding and slips.

A strong wind warning has also been issued for the Canterbury high country, with northwest gales ramping up to 120km/h on overnight Tuesday.

MetService is warning gusts are likely to be so powerful they will damage trees, powerlines and send unsecured structures flying.

It also advises driving may be hazardous, especially for high-sided vehicles and motorcycles.

In the North Island a strong wind watch is in force for Hawke's Bay, Wairarapa and Wellington on Wednesday while the Tararua Range and Mt Taranaki is under a heavy rain watch that day.