Severe storms hitting much of New Zealand look likely to bring even more trouble than first forecast.

Authorities have been flat-out preparing for what forecaster Canterbury Weather Updates called "the most severe looking set-up we've seen in a number of years". Hazards could include flooding, treacherous roads, and backed up sewers.

Forecasts for the storm have been progressively upgraded as the large and complex front moves over the South Island. MetService has forecast a thorough soaking with up to 55 millimetres of rain for Christchurch city during Friday and Saturday, an average month's worth, with the heaviest falls on Friday morning. Other parts of Canterbury will be wetter still, with up to 120mm of rain expected in Timaru and up to 200mm inland.

MONIQUE FORD/STUFF High seas in Cook Strait last week.

Gale force winds are also forecast, along with heavy snow at high altitude. The storms will affect most of the South Island and parts of the North Island.



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MetService called the storm "a significant weather event" and warned the combination of heavy rain and strong winds would cause widespread disruption to transport, localised flooding and slips.

"Winds of this strength can cause damage to trees, powerlines, unsecured roofs and structures, and make driving hazardous. Also in the South Island, wind chill due to cold temperatures and strong winds could cause stress to livestock," the forecaster said.

LISA-MARIE PAULETICH Methven after last week's snowfalls.

The Christchurch City Council is preparing for flooding with pumps in problem spots and warned some roads could close. Coastal flooding is also a risk because of high seas.

The council's land drainage manager, Keith Davison, said rain and floods could inundate the city's sewers, leaving some people's toilets, showers and sinks not working. People with this problem could contact the council for help with clean ups, he said.

The council would not provide sandbags though. Sports fields were likely to close on Saturday.

The Christchurch Transport Operations Centre advised travellers using South Island state highways to be prepared and check for warnings and closures. Heavy snow is expected on mountain passes including the Lewis Pass (State Highway 7), part of the main route north of Christchurch while the earthquake-damaged State Highway 1 is repaired.

Snowfields including Mt Dobson and Mt Hutt, where a metre of snow could fall, closed on Thursday due to high winds. Others are also likely to close.

The city council recommended highway travellers consider putting off driving until Friday afternoon or Saturday when conditions improve. "Those who do travel on Friday are urged to drive with caution."

The Ashburton District Council warned travellers to take care. It said that with a lot of rain already in the past few weeks, the latest falls would cause rivers to rise. Localised surface flooding could also affect roads.

Forecaster Blue Skies said the east of the South Island, from Mid Canterbury to North Otago, would bear the brunt of the storms. Christchurch would be very wet and windy, but flooding was unlikely to be major, the forecaster said.

Winds and high swells forecast for Cook Strait could affect inter-island ferry services.