Keep on the winter woollies - it's going to be a cold weekend.

There is flooding on one of the main routes through Hamilton, impacting Cobham Drive at the intersection with Grey Street.

In Auckland, one lane of the Mt Wellington off ramp is closed due to flooding. The exit can still be used, but drivers are asked to take care.

The heavy rain and strong winds that have battered Auckland and Northland this morning and early afternoon are moving across the country.

WeatherWatch analyst Philip Duncan says the weather is tracking southwest.

"Rain, and now these stronger winds these South to South West winds are going to cross over the upper north island through the rest of Saturday and Saturday night."

MetService has issued a severe thunderstorm watch for Greta Barrier Island, Coromandel Peninsula, Bay of Plenty and Gisborne until 10 o'clock tonight.

Duncan says the edge of the low will push temperatures down around the country tonight and tomorrow.

MetService meteorologist Rob Kerr Northland could see another 40mm-50mm of rainfall this afternoon, with rain likely to ease by 2pm, but in the Coromandel another 60mm-80mm could fall by 5pm.

"Auckland is covered by a watch that rain amounts could reach warning levels. It's going to get even colder, and it's going to produce snow to low levels and maybe even sea level for a time around the South Island.

"We've got what we call a subtropical low bringing in a lot of warm, moist air over the northern half of the North Island today and that's what's bringing a lot of that rain."

MAIN CENTRES FORECAST

Whangarei - Rain to heavy showers today, and a chance of thunder, winds turn to strong southwesterlies, a high of 19C. Expect showers on Sunday, with a chance of thunder and hail, strong southwest winds and a high of 16C.

Auckland - Rain with heavy falls, an easterly turns southwesterly with gusts of up to 90-100km/h, a high of 16C. On Sunday expect a few showers, strong southwesterlies and a high of 15C.

Hamilton - Rain with heavy falls today, turning to showers, winds turning southerly and a high of 14C. Showers on Sunday with the chance of thundery showers with hail, fresh southwesterlies and a high of 14C.

Tauranga - Rain, heavy at times with a chance of thunder, southwest winds and a high of 16C. On Sunday there will be fine spells, with a few showers in the afternoon, a strong southwest winds and a high of 16C.

Wellington - Cloudy today, with a northerly wind turning southerly in the afternoon, a high of 13C. Mainly fine on Sunday, late showers with a southerly change, a high of 14C.

Christchurch - Mainly fine today with a chance of an evening shower, light winds and a high of 14C. On Sunday expect a cold southwest change and afternoon with showers, a high of 15C.

Dunedin - Few showers for a time this afternoon, with southwesterlies and a high of 12C. On Sunday expect showers, turning to snow by evening, a southwest wind rising to gales, and a high of 9C.