Weather forecasters admit the finer details of the approaching storm have them at a loss.

The advances of science, multimillion-dollar technology and computer modelling allow them to say the shape and timing of the system is becoming clearer, but they cannot pinpoint the location of the worst of the expected rain, snow and gales.

MetService severe-weather forecaster Allister Gorman said there was no doubt a big low-pressure system would cover New Zealand from Sunday.

"There's a lot of potential for problems, with a very unstable atmosphere and cold air at very low levels, but the focus area for snow to low levels in the South Island is changing almost every [computer model] run," he said.

"It is always the trouble when you are looking at fairly small-scale things in the big picture. It's the same old position of New Zealand being surrounded by oceans, where computer models don't get much information."

Blue Skies Weather forecaster Tony Trewinnard said the system would bring rain, snow and gales.

"It is still an unusual weather system that we haven't seen the likes of for quite some time, so it is still very difficult to say which parts will see the worst weather," he said.

"It is the type of weather system that doesn't lend itself to precise prediction. It is good to say a week out there's a severe-weather system coming, but we can't be precise about where the worst weather is going to happen.

"We have to be prepared to say sometimes, 'I don't know'. That's the honest answer."

Gorman said extended wintry conditions were likely in the South Island from Sunday after a cold snap today, with snow to about 700 metres.

"Saturday will be a bit of a break day between what we get [today] and when it comes in again on Sunday. "There's a fairly good chance the east of the South Island is going to see decent snow quantities down to maybe 300m."

Snow warnings were issued for alpine passes last night and this morning. Porters Pass was expecting up to another 12 centimetres of snow at the summit by lunchtime today, on top of Wednesday's fall.