Heavy rain is set to sweep the northwest of the country and the upper South Island, with warnings of flooding and slips.

A heavy rain watch is in force for the Coromandel Peninsula, the western Bay of Plenty, the Gisborne ranges and parts of the upper South Island on Thursday.

“A front … slow moving just north of New Zealand is directing a strong, moist, easterly flow over the upper North Island,” MetService meteorologist Alasdair Holden​ said.

The front is forecast to move slowly south over northern New Zealand during Thursday and Friday, with periods of heavy rain and easterly gales forecast for parts of the upper North Island.

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MetService advised people to stay up to date with the latest severe weather information, as watches could be upgraded to warnings as the situation progressed.

The heavy rain watch for the Coromandel Peninsula ends at 2pm on Friday and a wind warning finishes at 1am Friday.

Bay of Plenty west of Kawerau, including Rotorua, is also in for some heavy showers, with periods of heavy rain from 4am-9pm on Friday.

MetService said rainfall amounts may approach warning criteria, mainly about the Kaimai Range as well as Rotorua.

“For the rest of the country, moving south, we’re expecting a few spots of light rain more or less for Rotorua and Taupō as well as Waikato,” Holden said.

A fairly complex low had come out of the subtropics, moving down over the North Island, he said.

“However, it has not played out as originally expected as there is a ridge of high pressure off to the south-east of the country that has dug in and has been holding it off.

“So we haven’t seen as much rain as we expected over those parts of the country. That includes Auckland as well.”

Westland was seeing “a decent amount of rain moving in from the north”, Holden said.

“A low out in the Tasman is extending a front that is growing in tropical air and that is being dragged down onto Westland at the moment, causing the rain we are currently seeing.”

Heavy rain watches were in place for Tasman and Westland until Friday night.

Wellington will be mainly fine, with some clouds that could increase in the afternoon.

Christchurch will also be cloudy and winds aren’t expected to ease until Thursday evening.

Auckland is forecast to see cloudy conditions with rain building on Friday morning.