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MetService is warning significant rain is possible about parts of eastern Otago today.
A spokesman said a slow-moving front would bring a period of onshore rain to eastern parts of the lower South Island today before moving away to the southeast.
A watch for heavy rain remains in force for eastern Otago (north of the Otago Peninsula).
Residents are advised to keep up to date with the latest forecasts in case parts of this watch are upgraded to a warning, or further areas are added.
A heavy rain watch is also in place for Westland and Fiordland north of Milford Sound.
Meanwhile, a cold snap will bring snow to parts of the country this weekend.
Weather warnings are in place for the Crown Range near Queenstown and the Milford Road in Fiordland, where snow could fall this evening.
MetService meteorologist April Clark said a "pool of cooler air" would move over the lower half of the South Island later today. Showers which fell above 1000 metres were likely to turn to snow.
"Obviously most people won't be out on the roads, and we're not expecting any towns to be affected.
"It is only if you are up on those highest roads, which I guess only essential workers will be on."
There is a risk of thunderstorms today and tomorrow on the west of the South Island. Thunderstorms are also expected to pass through Kapiti today.
"In places like Fiordland and lower Westland they'll be seeing intermittent, isolated thunderstorms, and their could be some good gusts with that and hail," Clark said.
"But again, no-one should really be out and about in Fiordland so most people won't see the worst of it."
Temperatures are expected to fall across the South Island. Inland places such as Alexandra could wake to frosts tomorrow morning. Overnight temperatures will fall as low as 3C.
The North Island is expected to be warmer - about 20C in Auckland - and showers will ease today.
"It will actually be fairly dry right into tomorrow," Clark said. "It is just western areas in the North Island that could get showers."
That weather pattern is likely to continue tomorrow, with mostly dry weather in the North Island, and temperatures in the low 20s in Auckland.
Showers are again expected in the west of the island, while the East Coast is expected to be fine.
Strong wind warnings are in place for the Canterbury high country, parts of Marlborough and the Wellington and Wairarapa regions tomorrow.
- additional reporting NZ Herald