The New Zealand Met Service has issued an alert level red for Canterbury, south of Ashley.

A Red Warning is reserved for only the most extreme weather events.

The significant heavy rain warning starts at 3pm tomorrow (Saturday 29th May) and extends till 11am on Monday 31st May.

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The Met Service expects 200 to 300mm of rain to accumulate about the Canterbury High Country and foothills during this time, with 80 to 120mm of rain farther east about the Plains and coast.

Peak hourly rates of 10 to 20mm/h are expected, especially from later Saturday and during Sunday.

They also advice the freezing level is forecast to be around 1400 metres on Saturday, then lifting to around 1800 metres on Sunday, before lowering to 1400 metres again on Monday.

Heavy Snow is expected above the freezing level throughout the event, but heavy snow may fall as low as 1000 metres in some places at times, especially on Saturday and on Monday.

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This rain is expected to cause dangerous river conditions and significant flooding.

Slips and floodwaters are likely to disrupt travel, making some roads impassable and possibly isolating communities.

Heavy rain is also forecast for other parts of central New Zealand, and possible severe gales for the South Island west coast.

For more information check the Met Services website or facebook page.