Northern parts of the North Island are likely to get a drenching tomorrow from the remains of the killer cyclone that left 17 dead in Samoa and inflicted severe damage on Fiji.

Cyclone Evan has been downgraded to an ex-cyclone but today the MetService issued a severe weather watch for Northland, Auckland, Coromandel Peninsula, Bay of Plenty, Rotorua and Gisborne.

In a mid-afternoon forecast, the Metservice said ex-Evan was expected to move east, just north of the North Island on Monday and direct a ''warm moist air mass of tropical origin'' onto the North Island.

"There is a high risk of heavy rain and a low risk of severe easterly gales in Northland," the weather agency said.

Moderate to heavy rain is considered likely for Auckland, Coromandel, the Bay of Plenty and in Gisborne.

"Watches have already been issued for heavy rain associated with this low and it is likely that warnings will be issued."

On Christmas Day, the low is expected to move east into the Tasman Sea.

On Wednesday the low would likely move southeast towards the upper South Island and there was a moderate risk of heavy rain in Westland.

Civil Defence and Emergency Management had been advised in Auckland, along with regional boat harbours and marina.

MetService predicted that in the 24 hours from tomorrow night, up to 150mm of rain could fall for coastal eastern areas north of Kawau Bay, including Omaha and Leigh.

Between Kawau Bay and Whangaparaoa, up to 100mm of rainfall could fall over the same period. Auckland could also expect up to 50mm at the same time

Showers and periods of rain were expected to continue through to December 27.

Winds were likely to reach gale force, between 63 and 87kmh, tomorrow and Monday, though severe, damaging gales were not expected.

With high tides currently relatively low, there was no expectation of coastal flooding.

"Northland and Coromandel are expected to bear the brunt of this system with higher rainfall totals expected than Auckland," the MetService said.

At sea there were warnings for oceanic areas of the subtropic and Pacific and coastal areas Brett, Colville and Kaipara.

For the Auckland VHF area there was a strong wind advisory for tomorrow morning in the area Bream Head to Cape Colville and for the afternoon in the Hauraki Gulf and Waitemata Harbour.

The moist northwesterlies ahead of Evan have bought thunderstorms to southern Hawkes Bay this afternoon - but no reports of damage.

- © Fairfax NZ News