A ridge of high pressure over the country is expected to move slowly eastwards during Tuesday and early Wednesday, says a MetService spokesperson.

"A front, preceded by a strong and moist northerly flow, is forecast to move onto southern New Zealand from the Tasman Sea late Wednesday, then move slowly northwards over the South Island during Thursday and Friday."

This front is expected to bring a period of heavy rain to the west of the South Island.

"There is moderate confidence of rainfall amounts reaching warning criteria about Fiordland during late Wednesday and early Thursday," says a MetService spokespersonl

"There is also moderate confidence of warning amounts of rain about Westland - mainly about the ranges - south of Otira during Thursday and Friday, but low conference for the remainder of Westland and Buller during the same period. There is low conference of warning amounts of rain about the headwaters of the Canterbury lakes and rivers on Friday."

In addition, the MetService advises that a period of north to northwest gales are also likely about exposed parts of southern and central New Zealand ahead of the front, but winds are not expected to reach severe gale force at this stage.