Rain has been hammering Auckland and other parts of the North Island overnight as New Zealand hunkers down for what forecasters predicted would be "one of the coldest weeks of the year".

The big chill started yesterday with cold south-to-southeast flows that spread north over the South Island.

This brought in a significant cold outbreak spreading north over the country, bringing bitterly cold southerly rain to the east of both Islands, as well as snow to low levels for inland parts of the South Island.

Heavy rain expected today

Meanwhile Aucklanders are again waking to heavy fog.

Fog restrictions are in place at Auckland Airport early this morning - with passengers being told to expect delays and cancellations.

So far 12 domestic regional flights have been cancelled and 13 have been delayed due to the fog.

Domestic flights from Auckland to Wellington, Christchurch, Dunedin and Queenstown have not been affected.

No international flights have been affected by fog at this stage.

Ferry passengers are being advised that the Devonport ferry services have been cancelled this morning due to fog too.

The Auckland to Devonport 7.15am service has been cancelled; as well as the Devonport to Auckland 7.30am service.

Rain hammered Auckland yesterday evening, carried through by the southeast flows, and is also forecast to come down in Kaitaia and Whangarei as well as Hamilton, Rotorua, Tauranga, New Plymouth Palmerston North and Wellington.

South Island road closures

There are heavy snow warnings for inland Canterbury. MetService issued a warning for up to 25cm above 300m in that area for the 25 hours from 1pm on Sunday. Today, starting as early as 1am, 20cm could fall above 400m in Marlborough south of Ward.

The widespread snow is "likely to significantly affect travel through these areas".

• In the South Island, the following state highways are currently CLOSED
  SH1 Hundalee, near the intersection with Claverley Rd
  SH1 Dunedin (Pine Hill) to Waitati
  SH8 Fairlie to Twizel (Burkes Pass)
  SH8 Tarras to Omarama (Lindis Pass)
  SH85 Kyeburn to Alexandra
  SH85 Palmerston to Kyeburn
• OPEN WITH RESTRICTIONS
  SH73 Arthur's Pass to Otira - Snow - Chains essential - Closed to towing vehicles
  SH73 Castle Hill to Arthur's Pass - Snow - Chains essential - Closed to towing vehicles
  SH73 Springfield to Castle Hill - Snow - Chains essential - Closed to towing vehicles.
• OPEN WITH CAUTION
  SH6 Fox Glacier to Haast - Strong Winds
  SH6 Hokitika to Fox Glacier - Strong Winds
  SH79 Geraldine to Fairlie - Snow and Ice
  SH80 Ben Ohau to Mt Cook - Snow and Ice
  SH87 Outram to Kyeburn - Snow
  SH94 Mossburn to Te Anau - Snow
  SH94 Te Anau to Milford - Strong Winds
  All other State Highways remain open.

On Sunday, snow briefly shut down SH85 from Palmerston to Kyeburn; however, by mid-afternoon, it had reopened. Additionally, there were warnings in force for SH1 between Dunedin and Palmerston, SH94 from Lumsden to Te Anau, SH6 from Kingston to Lumsden, and SH85 from Kyeburn to Omakau.

MetService forecaster Aidan Pyselman warned that this was going to be a "significantly cold week", with many parts of the South Island getting max temperatures of 5C or 6C.

Hauraki Gulf weather tweeted that they expected it to be the "coldest week of the year so far".

On Tuesday the cold southeasterly flow will bring more rain to central New Zealand, while a trough brings a period of rain to the North. While for the Coromandel Peninsula there is low confidence of significant amounts of heavy rain.

Wednesday will see the rain continue to affect central New Zealand as a strong southeasterly continues, and may also bring rain for Marlborough, eastern Wellington, Wairarapa, the Tararua District, Hawke's Bay and Gisborne.

Pyselman said a high-pressure ridge will form over the country starting on Thursday, pushing away the rain and snow, however, the low temperatures will remain, causing lots of frost around the country.