A severe weather watch has been issued for Bay of Plenty as a frontal rainband moves over the country.

The weather system is expected to bring heavy rain to central parts of the North Island overnight Thursday and during Friday.

The MetService is forecasting the possibility of heavy rain in the Bay of Plenty tomorrow. Image: MetService.

'A period of heavy rain is expected from late Friday morning through to Friday evening,” says a statement from the MetService.

Rain could reach warning criteria, especially about the ranges of eastern Bay of Plenty, however even some lower lying areas could see significant falls.

"The heaviest rain is expected about the ranges of northern Westland through to northwest Nelson and up to the Tararua Range, but other areas will also see some heavy downpours as the front moves up the country," says meteorologist Lisa Murray.

"This rain is accompanied by a risk of squally thunderstorms tomorrow afternoon for all western areas north of New Plymouth excluding Bay of Plenty, with localised downpours of 25-40mm in an hour possible.

"With many people still on holiday and involved in outdoor activities including tramping, care should be taken as large amounts of rain in a relatively short period will cause rivers and stream levels to rise rapidly," says Lisa.

Behind the front, the country sits in a disturbed southwest flow which eases on Sunday as a ridge pushes onto New Zealand, bringing more settled weather until early next week.

Meanwhile in the Tropics, Tropical Cyclone Ula is slowly making its way toward southern Vanuatu.

The expectation is for TC Ula's track to recurve southwards past New Caledonia early next week.

MetService meteorologists continue to watch this system closely to assess its potential to have any effect on New Zealand's weather

Colour enhanced Water Vapor satellite, #Himawari8. Green/white=moisture moving toward #NewZealand, red=dry air. ^CL pic.twitter.com/9Fmaxa1ZO8

— MetService (@MetService) January 7, 2016