"We have some big showers around the Auckland region already this morning and as we saw late yesterday, a still day with afternoon heating could trigger a few isolated downpours and thunderstorms this afternoon and evening.''
Yesterday afternoon a couple of fast-growing thunderstorms brought some minor flooding to suburbs in the south.
The clouds were rotating, but no funnel clouds or tornadoes were reported, Mr Duncan said.
The risk for isolated funnel clouds in Auckland today was moderate, with a low chance one could reach the ground and form a weak tornado or waterspout.