Rain had already been beating down on Auckland and New Zealand's north overnight.
A heavy period around 10am could further dump up to 25mm on Auckland before a short lull in the falls comes just after midday, the forecaster said.
Emergency services also spent last night monitoring heavy falls in Whangarei and eastern Northland amid concerns the rain could approach warning levels.
Whangarei residents woke to continuing rain early this morning, but that was due to ease in the afternoon as the city hit a top of 20C before another period of heavy falls was likely tonight.
A heavy rain watch was also now in place for the Coromandel, where Whitianga was expecting consistent falls right through Saturday along with a top of 18C.
Tauranga and the Bay of Plenty were also heading into a heavy rain warning period that was due to stretch through until 11am on Sunday.
Tauranga residents could expect a top of 16C and rainfall throughout Saturday with heavier bursts between 11am and 2pm, MetService said.
Rotorua could expect the rain to build throughout the morning as it headed for a chilly top of 12C.
The front bringing the rain was then due to slowly move down the country towards Napier and the Wairarapa coast where weather watches were in place from tomorrow.
Wellington residents, meanwhile, were waking to drizzling rain that should ease off through Saturday as the city headed for a top of 12C.
However, stormy, wet weather would hit the capital from tomorrow.
Blenheim and the Kaikoura Coast would also be subject to heavy rain watches from tomorrow as the stormy also hit the area.
Elsewhere in the South Island on Saturday morning, waking Christchurch residents could expect the odd shower and a top of 9C before patchy drizzle took over in the evening.
Dunedin could also expect the odd shower and drizzle and a top of 7C.
Queenstown, meanwhile, was expected to remain dry but residents were waking to frosts and a freezing -1C temperature, MetService said.
