Heavy rain and severe gales are possible for large parts of the South Island from late today through to Wednesday.
MetService said a front preceded by a strong northwest flow is behind it issuing a number of weather warnings and watches.
"The front should continue moving north over the South Island during the remainder of Monday and through Tuesday, followed by a cold south to southwest change," the forecaster said.
"A deep low may develop along this front to the east of the lower South Island on Tuesday, bringing the potential for severe gale southerlies, heavy rain and large southerly waves for eastern parts of the South Island.
"There is some uncertainty with the forecast position and track of the low at this stage, and forecasts may change as the event draws nearer."
An orange heavy rain warning is in place for the headwaters of the Canterbury lakes and rivers, about and south of Arthur's Pass from noon on Monday to midnight Monday. MetService said up to 160mm of rain can be expected on the main divide and up to 110mm within 15km east of the divide.
The same warning is in place for Westland about and south of Otira from 7am on Monday to 7am on Tuesday. People can expect to see up to 260mm accumulate about the ranges and up to 120mm near the coast.
Fiordland is under the same warning from midnight Sunday to noon on Monday. MetService said up to 120mm of rain can be expected.
Thunderstorms are possible for all of these areas, MetService warned.
And Dunedin is under an orange heavy rain warning from midnight Monday to noon Tuesday, with up to 100mm of rain on the way.
Meanwhile, a heavy rain watch is in place for Southland and Clutha from 8pm on Monday to 8am on Tuesday. Stewart Island is covered by the watch from 8pm on Sunday to noon on Monday, while Westland north of Otira and Buller is under the watch from 8pm on Monday to 3am on Tuesday.
Canterbury excluding Banks Peninsula is also under the heavy rain watch, from 7am to 7pm on Tuesday. Banks Peninsula is covered by the watch from noon on Tuesday until noon on Wednesday.
The headwaters of Otago lakes and rivers is covered by the heavy rain watch from 7am to 8pm on Monday, while Otago excluding Clutha is under the watch from midnight Monday to noon on Tuesday.
The forecaster said thunderstorms are possible in parts of Westland and the headwaters of the Otago lakes and rivers, while the other areas can expect periods of heavy rain which may approach warning criteria.
An orange strong wind warning is in place for Canterbury High Country from 3pm Monday to 3am Tuesday: "Strong wind gusts could damage trees, powerlines and unsecured structures. Driving may be hazardous, especially for high-sided vehicles and motorcycles."
MetService has issued a strong wind watch for Canterbury, North Otago and Dunedin from 7am on Tuesday to 7am on Wednesday. The same watch is in place for Southland from 9pm on Sunday to 6am on Monday.
For Canterbury, North Otago and Dunedin, severe gale south to southwest winds are possible, while in Southland northwest winds may approach severe gale in exposed places.
Wellington and Wairarapa south of Masterton is under a strong wind watch from 8pm Monday to 6am Tuesday and Marlborough is under the same watch from 6pm Monday to 3am Tuesday.
People are advised to keep up-to-date with the latest forecasts on the MetService website in case any changes are made or further areas are added.