Sydney may be in for a stormy Monday with the possibility of two bouts of thunderstorms, and areas of heavy rain.

On a day when NSW Premier Mike Baird joined State Emergency Services and insurer NRMA to warn the state to prepare an active thunderstorm season, Sydney has the chance of both an afternoon and evening storm. St Ives topped the list of the 10 most storm-impacted suburbs over the last financial year.

The first wave came through the city in the early afternoon, with another system due to arrive ahead of a cool change by about dinner time, meteorologists at Weatherzone said.

The Bureau of Meteorology has updated its severe thunderstorm warning to cover a region north of the Hawkesbury River, almost up the Queensland border.