If spring cleaning has lately given way to mould busting in your house, you're not alone.

Sydney is in the midst of its most humid October in at least 15 years, according to Brett Dutschke, senior meteorologist at Weatherzone.

"It's been more typical of summer [and] definitely unusual for October," he said.

A lightning bolt strikes as a storm cell is seen above the Sydney Harbour Bridge on Saturday Credit:AAP

Afternoon humidity levels have been averaging about 70 per cent compared with the October average of 55 per cent, Mr Dutschke said, citing readings from Sydney Airport.