"The rain might not reach the warning level criteria of 50mm within six hours, but we're looking at a shorter period [of time]," Ms Flynn said.
A cold front crossing the upper North Island may mean strong southwesterly winds follow the rain.
Severe weather warnings have also been issued for the Coromandel, Northland and Bay of Plenty.
Winds in the Coromandel are likely to reach at least 40km/h, with some gusts exceeding 60 km/h in exposed places.
Ms Flynn said wind speeds will pick up following heavy rain between 5pm and 9pm.
"It's a bit later than Auckland so hopefully people will have left work already.
"It's not a severe situation because the winds strengthening into the evening aren't happening at the same time as the rainfall."
Road conditions will be hazardous and drivers are warned to take care.