MetService is calling it a "significant rain event".
There is an increased likelihood of escalation to a red heavy rain warning on Wednesday morning.
Here are the latest developments.
NIWA said some rainfall totals may reach one to two times the April normal.
New Zealand is set to be slammed by subtropical moisture.
At this stage, it looks like the western, inland and northern South Island will see the heaviest rain.
The nationwide average temperature was 14.8°C.
A heavy rain watch has been issued for the ranges of Westland District.
Some places haven't been able to escape showers.
