Heavy rain is pummelling parts of the country, causing widespread travel disruption, flooding and evacuations.

The top of the South Island was bearing the brunt of the rain, with state of emergencies issued as a result of significant rainfall. Dozens of homes have been evacuated due to rising floodwaters.

Fire and Emergency said it had responded to 31 calls, with crews rescuing people trapped in vehicles, assisting with evacuations and pumping water from flooded homes.

Meanwhile, Auckland had been hit by severe thunderstorms and torrential rain.

Here's what you need to know.

A state of emergency was declared for the Nelson Tasman region as a result of severe weather and evacuations.

Tasman District Council said the declaration would allow agencies to rapidly respond, with slips and flooding across the region and further rain forecast.

People were being evacuated in Brightwater and Upper Moutere and residents in Riwaka were expected to have to leave.

A local state of emergency was also in place for part of Marlborough, where 60 households in Spring Creek near Blenheim were ordered to evacuate because of fears a Wairau River stopbank might not hold.

Fire and Emergency (FENZ) had activated its Local Coordination Centre in Nelson to coordinate its response activities across the whole top of the South area.

An emergency mobile alert was issued this afternoon to Tapawera and Tadmor Valley near Nelson.

Nelson Tasman Emergency Management said flooding from the Motueka River and Tadmor River would impact the Tadmor Valley and Tapawera area.

It was recommending people evacuate to higher ground immediately.

MetService issued several severe weather warnings and watches across the country.

A red heavy rain warning was in place for western Bay of Plenty, Tauranga, Whakatāne and Rotorua.

Earlier parts of Northland, Auckland, Waikato and Bay of Plenty were under a severe thunderstorm warning, however these had been lifted.

Meanwhile, a heavy snow watch had been issued for Central Otago, Southland north of Lumsden, also inland Dunedin and Clutha until noon Saturday.

Orange heavy rain warnings:

Heavy rain watches:

The top of the South Island was cut off with a number of state highways closed.

NZ Transport Agency was warning drivers to be prepared for wet weather driving and to take extra care on the roads.

FENZ Steve Trigg warned people not to drive through floodwater and to respect all "road closed" signs.

"It's impossible to judge the depth of the water, so when people become stranded they are in danger of drowning."

Here is a list of the highways closed:

Roads closed in Marlborough include:

Dozens of domestic flights bound for or departing from Auckland Airport were cancelled or delayed due to severe weather conditions.

Auckland Airport said 21 flights had been cancelled and 11 flights delayed.

It said the weather was mostly impacting flights to or from regional destinations.

The airport also confirmed that a China Eastern Airlines flight from Hangzhou to Auckland was diverted to Christchurch due to the unsettled weather conditions. China Eastern Airlines also confirmed the diversion.