More snow and heavy rain are set to fall today, as the North Island remains in the grip of a bone-chilling storm.

Thousands of households are without power and roads across the centre of the island are buried in snow as the storm unleashes its fury.

Rescuers today hope to reach two groups of trampers trapped by swollen rivers in the Tararua ranges and in the Rimutaka Forest Park.

Air passengers across the country are also experiencing major disruptions, and scores of people were forced to sleep overnight in the Wellington Airport terminal.

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This morning the New Zealand Transport Agency (NZTA) pleaded with motorists to keep off central region roads until they were safe to travel on.

Wellington is mopping up after a night of heavy rain, and slips and flooding are causing problems throughout the region this morning.

The MetService is forecasting up to 30cm of snow across the Central Plateau, Hawke's Bay and Gisborne ranges for the next 12 hours.

Police say the southern Wairarapa township of Martinborough has been largely cut off by flooding, though the NZTA says there is still one road out.

Public transport in the capital is still hampered by stormy weather. All ferry sailings across the harbour are suspended and trolley buses operating on batteries are delayed.

Waterlogged central and eastern regions of the island can also expect more rain today, and forecasters warn it will be heavy.

"Above about 800m we expect further accumulation of snow especially about the Desert Rd - a further 20-30cm of snow, " said MetService lead forecaster Michael Martens.

He said the worst had now passed over the capital.

Highways across the central North Island are not expected to reopen until later today. Porters Pass is the only South Island alpine highway still under a road snow warning.

Emergency welfare centres across the Ruapehu District would reopen today and council officials would be moving through townships to make sure people were safe and had basic needs.

Powerco said several thousand customers went without power last night after heavy snowfalls downed lines and flooding created havoc across the company's network.

Powerco network operations manager Phil Marsh said damage was spread across a large swathe of the lower North Island including Wairarapa, Manawatu, Whanganui, Taranaki, Ohakune, Waiouru and Taihape.

Crews would try to repair the broken network from first light.

NZTA asked motorists for a second day running to only travel if it was urgent. The only routes linking Auckland to Wellington was around the edges of the island.

Staff and equipment from the upper North Island had been called in to clear roads.

Motorists were advised to slow down and watch for slips and rockfalls triggered by melting snow.

Flights had resumed out of Wellington Airport this morning but travellers were warned to expect delays as the backlogged was cleared.

Snow warnings in the South Island had now been lifted as snow showers eased and strong winds warnings had been lifted for Kapiti and Whanganui.

Today's temperatures

Auckland:

Showers, gales, easing tonight. 12C

Hamilton:

Showers becoming fine. Southerlies.11C

Tauranga:

Showers become fine. Southerlies. 12C

Taupo:

Rain clearing this afternoon. Fresh southerlies. 8C

New Plymouth:

Rain becoming fine by lunch. Strong southerlies. 12C

Napier:

Heavy rain easing to showers. Strong southerlies.12C

Wellington:

Rain turning to showers, gales easing. 10C

Christchurch:

Overcast, showers then clearing tonight. 9C

Dunedin:

Cloudy, showers clearing tonight. 8C