But it won't be anywhere close to the substantial amount desperately needed in tinder-dry areas of the North Island, recently classified as experiencing a "large-scale adverse event".
"Rain starts to develop in the far North tomorrow night, but it's 15 hours' worth of rain - it's a bit of a flash in the pan," Ngamanu said.
But strong winds, even gales are set to batter the northeast of the North Island, with a severe weather watch issued for Tuesday and early Wednesday.
The MetService watch has been issued for Northland and Auckland from northwards of Warkworth, Great Barrier Island and Coromandel Peninsula.
Southeasterly winds could reach severe gale strength in northeastern parts of the North Island, clocking up to speeds of 75km/h.
Cyclone Gretel will whip swells up to 6m high along the northeastern coastline, from eastern Northland through to the Bay of Plenty on Tuesday.
Gisborne surfers can expect 5m swells on Tuesday, expected to ease over the following days, Ngamanu said.
MetService has issued a strong wind watch for Bay of Plenty east of Whakatane and Gisborne for Tuesday to Wednesday morning, with strong southeasterly winds possibly becoming severe gales in exposed places.
A "few showers" are expected above Wellington, Wairapara and Hawke's Bay tomorrow, Ngamanu said.
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Cyclone Gretel is expected to be reclassified as a deep subtropical low on Tuesday as it moves over waters to the northeast of New Zealand.
But as it still evolving, Ngamanu recommended people to stay up to date with the latest MetService forecasts, with another due on Monday morning.
Gretel is expected to miss the South Island completely, but the island is set to get its own share of rain, as a wet front works its way up the island.
But as it's a fairly weak front, most of the rainfall is expected to shower western areas of the South Island overnight tonight, Ngamanu said.
A few showers in the east of the South Island are expected to clear Monday morning and continue to clear as Cyclone Gretel brings rain to the North.
But cooler temperatures are on the way for Southlanders, as another ridge spreads on to the Tasman Sea, Ngamanu said.
"It will bring cooler weather than what has been experienced for the last few days."