A number of heavy rain watches are in place around the motu on Thursday.
The MetService said a complex trough would cross much of northern and central New Zealand on Friday and would be preceded by rain and strong northeasterlies for many places.
MetService meteorologist John Law told Morning Report there could be bursts of "fairly intensive" rain accompanying thunderstorms on Thursday evening and into Friday.
"Could be some really intense bursts through there, 20-30ml of rainfall not out of the question as [the thunderstorms] run through."
Law said people should keep a close eye on the weather forecasts.
MetService was now working closely with the National Emergency Management Agency to ensure the weather watches and warnings were easy to use. That included a note about whether a warning was likely to be upgraded, he said.
"[This front] takes its time to clear through on Friday, there is some drier weather coming back towards us on Saturday with the worst of the rainfall now looking to be staying off down towards the far southern side of the country.
"It's going to be a one to keep a close eye on but I think the worst of the weather hopefully through Friday and things looking a bit better as we head into the weekend."
Fiordland north of Doubtful Sound and the ranges of the Westland District were among the first expected to get periods of heavy rain with amounts which might approach warning criteria, though that chance was low.
The heavy rain watch there kicks in at 11am Thursday and runs til the early hours of Friday.
The forecast for Northland from 1pm Thursday was for periods of heavy rain and thunderstorms with possible bursts of heavy rain are possible, and a moderate chance of upgrading to a warning.
Auckland, including the Coromandel Peninsula, could expect periods of heavy rain and thunderstorms from 8pm, while the same weather was likely to reach Bay of Plenty and Rotorua by 10pm.
Heavy rains were also expected near Mount Taranaki and in the Tasman region this evening.