New Zealanders will get a "shock to the system" this week as temperatures drop around the country.

Much of central and southern New Zealand was on some form of weather watch or warning because of a low-pressure system making its way up the South Island, Metservice meteorologist Andrew James said.

It brings with is heavy rain in the west and strong north-westerly winds.

Heavy rain warnings are in place for Buller, Westland and the Canterbury lakes and rivers headwaters, and a strong wind warning is in place for Canterbury high country, with expected gusts up to 120km/h in exposed places.

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Strong wind watches extend to Wellington and Wairarapa until late Tuesday morning.

While it's all rain and bluster for those parts of the country, the north-westerly winds "tend to bring warming", James said.

Unfortunately, as that system moves on, temperatures in some areas are expected to plummet. Hastings, for example, will go from a high of 23C on Tuesday to an overnight low of just 3C.

"After the front goes through, it is cold in behind . . .it's going to be a bit of a shock to the system there," James said.

"There's a number of places where there's single-digit, close-to-zero temperatures overnight."

Despite the below-average temperatures, the weather is expected to be "more settled", with partial cloud in the west and fine weather in the east from Wednesday.

"The further north you are, the better the weather gets," James said.

The "changeable" weather - going from rainy but warm, to fine but cold - was common in spring, he said.

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The forecast for Monday

Auckland:

Morning cloud with some showers clearing to fine spells. High 16C, low 10C

Hamilton: Morning cloud clearing to fine spells. High 16C, low 8C

Wellington: Fine in the morning but cloudy in the afternoon with winds rising to gale force in the evening. High 15C, low 11C

Christchurch: Mostly fine but high cloud increasing and strong winds. High 19C, low 8C

Dunedin: Fine spells with some showers and strong winds. High 15C, low 6C