South to southwest swells of 6m to 8m are expected today and Tuesday for nearly all of the South Island, and the huge waves will continue to spread up the coasts of the North Island.
Meanwhile, a brief hail storm hit Christchurch earlier this afternoon and snow is falling across the lower South Island as a blast of frigid air moves on to New Zealand from the Antarctic.
Southwest gales in central and southern areas mean the wind chill factor is expected to get as low as -20 deg C.
Christchurch saw one of its “first salvos” as the hail came down with more heavy rain and hail forecast later, the MetService said.
The first front bringing hail would clear before another was expected to bring the showers back, the MetService said.
Temperatures are forecast to reach 1 deg C overnight in Christchurch.
A heavy snow watch was put in place for the Banks Peninsula between 2am and 2pm Tuesday where snow was expected to reach as low as 200m, “getting heavy at times”.
Waves of up to 6m are forecast for Wellington's south coast from Tuesday morning, and residents on the coast are urged to take care.
For Wellington, MetService issued a warning for the 41 hours from 6am on Tuesday to 11pm on Wednesday.
The southerly swells were due from Baring Head to Sinclair Head at Ōwhiro Bay, and the biggest risk period was due to be high tides on Tuesday 8.24am and 8.54pm and on Wednesday at 9.18am and 9.48pm. Swells would ease by Wednesday, dropping to around 4 metres.
Interislander sailings for Tuesday have been cancelled, and Wednesday cancellations are likely.
The 10.30pm Kaiarahi sailing from Picton on Monday will be the last before the cancellations come into effect, Kiwirail said.
"These are very large, high-energy waves, and can catch people unawares if they're walking on beaches or driving along coastal roads. There's also the potential for waves to affect land or property near coasts, MetService meteorologist Alwyn Bakker said
The winds driving the swell are also a potential risk, MetService said. Exposed places on the south and east coasts of the South Island may have severe gales at times, while parts of Nelson, Wellington and the Marlborough Sounds, and the east coast of the North Island from the Wairarapa to Napier, are also at risk.
Wellington Region Emergency Management Office is preparing advice to issued on Monday afternoon.
Huge waves pummelled the south coast last April damaging homes and leaving one person in hospital. In response to the extreme weather event MetService introduced a wave warning system for Wellington's south coast and harbour.
The wintry weather has already disrupted flights in lower South Island, with Air New Zealand services in and out of Invercargill cancelled as gale force winds and sleety showers hit the city.
Snow fell as low as 200 metres in parts of Southland and Otago and more was expected on Tuesday morning.
Meanwhile water tankers have been deployed in three urban centres in Horowhenua after flooding at the weekend overwhelmed water treatment facilities.
Five water tankers have been sent to Levin, Shannon and Tokomaru to help people conserve water.
Food supply businesses, restaurants and industrial water users have been asked to reduce using water or shut as supplies of clean drinking water in the district reach critical levels.
- RNZ and Star News
