Surface flooding and slips could happen, which could make driving dangerous. Motorists should drive according to the conditions.
The rain is expected from Saturday to Sunday morning.
MetService is forecasting up to 150mm of rain, with the largest downpour expected north of Gisborne city.
The highest rainfall is expected on Saturday between 3pm and midnight.
"While the wettest spots this weekend are likely to be in eastern areas of the North Island, nowhere in the North Island is guaranteed to stay dry," Glassey said.
From Waikato to Wellington, along the west of Te Ika-a-Māui the North Island, scattered rain should ease to showers north of Taranaki.
The South Island, on the other hand, should have mostly dry weather as a ridge of high pressure moves across the island.
Canterbury, Otago and Southland can expect low cloud and fog in the mornings in some areas, with patchy drizzle around Banks Peninsula.
Glassey said: "Mostly fine weather is forecast over the South Island until Monday, with only isolated showers for northern and eastern parts."
A low will move over the country from the Tasman Sea, bringing wet and windy weather for the start of the week.
The low should move east, pulling cold air from the south over the South Island.
More snow could fall in inland parts of the island, but not as low as this week.
It comes as wintry conditions extended into spring, with snow falling in Dunedin and Christchurch on Monday and Tuesday.
Snow fell on the mountain passes across the South Island, as well as in Hawke's Bay.
On Tuesday, Auckland recorded its coldest night of the year, as Niwa measured a 1.8C temperature at Western Springs.
While some lament a lingering winter, Glassey said, "Changeable weather is typical of spring and fluctuations in temperature are expected.
"It is not uncommon to get warm, settled weather followed by cold outbreaks at this time of year."
