A subtropical low moving south towards New Zealand is expected to bring widespread heavy rain, strong winds and unusually large seas from midweek, with impacts likely to be felt across much of the country by Friday.
Forecast heavy rain and severe easterly gales have triggered weather watches for Northland, lasting from Wednesday through until Friday.
MetService meteorologist Alanna Burrows told 1News the weather watches issued for Northland were "just the beginning".
"As they get closer to the event, more watches and warnings are likely to be issued."
The heavy rain watch was in place for Northland for 53 hours from 10am Wednesday. A strong wind watch had also been issued.
Overnight Wednesday into Thursday, the system was expected to push further south, with rain spreading into Auckland, the Coromandel Peninsula, and northern and central parts of the North Island.
By Friday, heavy rain was expected to extend into the upper South Island, including Nelson/Tasman, with further rain possible for Westland, Bay of Plenty and northern Gisborne/Tairāwhiti, while strong winds may also affect parts of Taranaki, inland central North Island and Banks Peninsula.
The system had a moderate chance of developing into a tropical cyclone over the eastern Coral Sea on Monday and Tuesday morning before conditions become "unfavourable" as it approaches New Zealand, Burrows said.
"Even if it doesn't get named, it's still got a lot of moisture that's collected in the tropics, quite a lot of warm air wrapped around in that, so we've got quite a prolonged weather event. This system is picking up a lot of moisture, so we're expecting a lot of rain."
'Quite unusual': Northerly swell to bring large waves
The system was also expected to generate dangerous marine conditions, with unusually large northerly swells forecast.
New Zealand typically has a prevailing southwesterly swell, but Burrows said the low coming from the tropics was bringing "large northerly waves" along with it.
"For areas like Northland, you could see some heavy swell. On Thursday, you might see some waves and swell waves over 4m, so that's quite unusual from the north."
The West Coast of the South Island could experience waves exceeding 6m while the swell could get up to 3m along the west of the Coromandel Peninsula, she added.
Full list of weather watches and warnings
Heavy Rain Watch
Northland - 53 hours from 10am Wednesday to 3pm Friday
Strong Wind Watch
Northland - 48 hours from 6pm Wednesday to 6pm Friday
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