It might pay to keep an extra layer for your bed or a winter jacket handy this week as a cold snap brings with it the first signs of winter.

Snow has made an early arrival in New Zealand, accompanying cold air that has spread northwards across the country.

Rain and cooler temperatures are forecast for the Bay of Plenty this week. Photo: File.

Overnight temperatures across parts of the South Island dropped down below freezing, with the lowest recorded at Lake Tekapo at a chilly -2C.

But the cold air has not made it right the way across the rest of the country yet, with temperatures in the far North only dropping to 15C. In the Bay of Plenty it's looking like a mild week of temperatures between 16 and 19 degrees.

This week sees the southerly winds dominate, continuing to bring some wintry showers and colder temperatures. However, temperatures are expected to pick up again towards the weekend.

'We really have seen a change in the weather over the last week,” says MetService meteorologist John Law.

'Parts of the country were exceptionally warm - Auckland airport for example, reached 25C on Friday - but this week we are looking at unseasonably cold temperatures and snow to low levels for parts of the country.”

He says the change in weather is due to the arrival of a cold air mass from the Antarctic, which is set to continue moving northwards.

As well as the colder weather, thunderstorms also feature strongly in the forecast for Tuesday, bringing with them some hail to parts of the country, especially the west of the North Island and the far east of the South Island.

In addition to providing a significant wind chill factor, the stronger winds forecast during Tuesday are set to help generate high swells around coastal parts of New Zealand.

Both Tauranga and Whakatane are forecast to get the occasional shower today and Wednesday before a fine, dry spell arrives to end the working week.

However, Metservice's long range forecast doesn't paint a pretty picture at this stage, with rain predicted to return over the weekend the start of next week.

It is looking very similar further up the East Coast, with Thames and Whitianga predicted to get some rain along with temperatures between 16 and 21 degrees.

The MetService Severe Weather forecasting team has issued an alert for heavy snowfall across parts of Southland, southern Fiordland and Otago.

The heaviest snowfall, around 15-25cm, is forecast above 600 metres, but some snow could be seen as low as 100 metres for parts of the South Island.

Road snowfall warnings have also been issued for the higher passes of the South Island, while snow is also forecast about the top of the Desert Road.