Weather: Half a metre of West Coast rain could be followed by lower South Island frosts

Up to half a metre of rain could be dumped in the West Coast over the weekend, followed by the high risk of frosts in some lower South Island areas.

A "significant rain event" has been impacting the West Coast from Friday to the end of Sunday, with WeatherWatch warning there could be with rainfall totals of 250mm to 300 at the coast and potentially half a metre of rain (500mm) in the mountains.

"The bulk of the heaviest rain will be in the middle section of the West Coast and further southwards - or from about Hokitika southwards. Rainfall totals dramatically drop off the further north you go, which is better news for places like Greymouth and Westport," it says.

"Franz Josef is one area that is most exposed with forecasting at least 270mm there over the next three days, but totals further inland may exceed 500mm, which is a huge amount of rain falling in fairly short period of time. This may well lead to slips, flooding and some road closures."

MetService has issued a heavy rain warning for Westland from Otira southwards, with periods of heavy rain forecast to continue through to Sunday evening.

"Expect a further 250 to 350 mm of rain to accumulate about the ranges on top of what has already fallen, bringing rainfall totals for this event to 400 to 500 mm or possibly more. Coastal areas can expect a further 100 to 150 mm of rain to accumulate during this time," MetService warns.

Meanwhile, a heavy rain watch remains in force for northern Fiordland, and heavy rain watches are also now in force for the Otago and Canterbury headwaters, Buller, northwest Tasman and northern parts of Marlborough.

Following the risk of heavy rain, slips and floods comes the risk of cold nights and even frost. 

"It was 20C overnight in Auckland - tough for sleeping! But check out what's about to happen in the lower South Island!" WeatherWatch says on Twitter.

"Frost risk for some in the lower South Island tomorrow night (and Monday night). Not quite 'scorching'."