Cold Antarctic air is sweeping up the country, causing temperatures to plummet and bringing ice and snow to 300 metres - and forecasters warn there's more bleak stuff to come.

"Nature is giving us the first taste of winter," MetService meteorologist Tui McInnes said.

Just last week Auckland was basking in warm temperatures in the low 20s, fuelled by a humid northerly flow.

Many places across New Zealand have even enjoyed their warmest May on record.

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However, in just the past two days there had been a noticeable change in temperature right across the country.

"In behind the big thunderstorm event a couple of days ago has been a lot of cold air, which has come up from quite far south, in the southern ocean," McInnes said.

"It has been dragging temperatures down over New Zealand, making quite a stark contrast for everyone."

This morning a bitterly cold southwest breeze is blasting up the country, bringing snow down to 300m in the far south, and to parts of the central North Island.

Over the next few days Auckland is forecast to sit around a maximum of 16C through the day, before dropping to 14C on Monday.

Night-time temperatures see an even more dramatic drop, from an average low of 12C tonight dropping steadily to 6C on Monday.

Next week is looking like much of the same.

"That drop will definitely be noticeable," McInnes said.

"It might be time for the electric blankets and hot water bottles."

Elsewhere in the North Island is forecast much of the same, while most of the South Island will see daily highs around the low teens through next week, and night-time lows approaching 0C.

Tips to help stay warm

EECA Energywise technical expert Christian Hoerning has provided some tips to keep warm this winter:

Insulation

Well installed insulation that helps keep the heat in during winter is the priority. Insulation makes your house easier to heat properly, and more comfortable and healthy to live in. Colder houses place greater stress on older people, babies and the sick and are more likely to have mould that causes respiratory symptoms.

Insulation works by slowing heat loss from your house, making it easier to warm. If your house is not insulated, good quality, well-installed underfloor and ceiling insulation could halve your house's heat loss.

If you have insulation, it's worth checking to see if it is up to scratch. Old insulation may have settled, become damp or have gaps and no longer perform well. Sometimes insulation also gets moved around when people are doing work in the roof space or under the house.

Government programme Warmer Kiwi Homes offers grants from July 1 covering two thirds of the cost of ceiling and underfloor insulation, as well as ground vapour barriers. Additional contributions from community organisations will be sought to make the retrofits as low cost as possible.

Grants covering two thirds of the cost of heating appliances will be available from July 2019.

Heating

For larger rooms you want to heat regularly like a living room, it's worth paying a bit more upfront for a fixed heater with lower running costs and more heat output than an electric heater. This could be a modern wood or wood-pellet burner, an energy-efficient heat pump, or a four-star flued gas heater. Electric heaters may be enough for smaller rooms and rooms you only heat occasionally, like bedrooms - they're cheap to buy but more expensive to run.

Curtains

Lined floor-length curtains can reduce window heat loss significantly. To work effectively, the curtains need to create a reasonably good seal to trap the air in the gap between the curtain and the window – if they are installed with gaps around their edges, air can circulate freely and won't insulate much.

There are curtain banks in many areas that offer free curtains to lower-income households.

Ventilation/moisture reduction

Ventilation is important, but getting it right is a balancing act: if there is not enough ventilation, air quality won't be maintained and moist air will not be able to escape. However, too much ventilation means a draughty, cold house.

Control moisture

Start with moisture hot spots: kitchens, laundries and bathrooms. Install extractor fans that vent outside (not into your roof cavity) to take away moist air. If you can't install extractor fans, open the windows when you've been cooking, showering or drying clothes to let moisture escape that way.

Control draughts

Follow this up with some simple draught stopping. Fill any gaps in floor boards and around window and door frames. Install weather stripping on the inside of window and door frames.

Air the house

Finally, make sure your house gets a regular airing – a quick blast of fresh air through the whole house a few times a day should be enough. Home ventilation systems are a convenient alternative but can be expensive – just airing the house regularly yourself should achieve all the ventilation you need.

Low-cost tips for saving energy in winter

• Shut your curtains at dusk to keep the heat in.

• Use window insulation film. This is a cheap, easy way to insulate windows, although it's not as durable as actual double glazing. Used in conjunction with well-fitted, thermal backed curtains, it is a good, low-cost window insulation option.

• Only heat the room you're using. Keep the door closed and use door snakes to keep the heat in.

• If you have a heat pump, use its timer and thermostat and clean the filter regularly. No matter how great the specs, a heat pump cannot perform efficiently if you don't look after it or leave it on all the time at full bore.

• If you have a fireplace that isn't used, prevent draughts by filling a large plastic bag with crunched up newspaper (make sure people know they can't light the fire).

• Replace standard light bulbs with LED bulbs.

• Install a cylinder wrap around your electric hot water tank and insulate the first few metres of hot water pipe.

• Install an efficient shower head.

• Wash your clothes in cold water.