It comes just days after Niwa released its three-monthly outlook predicting wetter and warmer conditions for most, with a possible flooding event in the second week of July.
It highlighted the plumes of tropical moisture threatening to bring a deluge of rain to New Zealand, with signs pointing to it happening in the coming five days.
MetService said a couple of low pressure systems were set to affect New Zealand this week.
While there would be a decent start to the week it warned "things turn to custard midweek".
"Those north of about Westport should expect a few damp days from Tuesday," the forecaster tweeted.
At this stage the worst of the stormy weather would impact the top half and western regions of the North Island and the top of the South Island.
In its severe weather outlook it warned of gales buffeting the North Island and heavy rain for western regions from Tuesday and northern regions coming under fire on Friday.
Later in the week a bout of cold air was expected to bring more snow to the South Island, affecting alpine passes and higher roads in Canterbury and Otago.
The prospect of a deluge in coming days follows Sydneysiders on high alert as flood waters rise, with orders to evacuate homes as a monster storm bears down on New South Wales.
Forecasters have said the worst of the weather is yet to come with an East Coast Low set to near the coast on Sunday and drag into Monday.
Residents living in Sydney's west, southwest and southern suburbs are in danger of flooding with predictions as much as half a metre of rain could fall across the city in the coming day.
