The worst affected areas will be Coromandel Peninsula, Bay of Plenty west of Te Puke, Gisborne and Hawke's Bay north of Napier.
MetService is warning torrential rain will start falling from 9am tomorrow and last up to 18 hours in some areas.
Forecasters are warning it is possible that rain will reach warning levels with up to 90mm of rain falling on Gisborne and Hawke's Bay between 6pm Saturday and 12 pm Sunday.
Coromandel and Bay of Plenty could get as much as 70mm in 12 hours.
Forecaster Claire Flynn said the low-pressure system that was now starting to hit the Far North would move slowly down across the island with a chance of thunderstorms hitting parts of Northland and Great Barrier Island from this evening.
There was also a chance of thunderstorms across the central North Island today due to daytime heating, said Flynn.
Rain would fall across the top half throughout the day with thunderstorms expected to strike eastern parts from Kerikeri to Tauranga on Saturday.
"It will be a rainy day for the north of the country on Saturday," said Flynn.
Rain and showers would continue through to Sunday but the weather would start to improve across the island, she said.
Meanwhile torrential rain is expected to continue to drench Banks Peninsula today before easing off early on Saturday morning. The weather would gradually improve on Sunday as the low moved away to the east.
The West Coast of the South Island would enjoy a fine day tomorrow before the weather deteriorated on Sunday.
