The system is expected to move east, and will affect Manly and the northern beaches from 4:45pm onwards.
Forecasters warn heavy rainfall may cause flash flooding, and residents should also brace for destructive winds.
The State Emergency Service recommends moving your car under cover or away from trees, and securing loose items around your house, yard or balcony.
Residents are also advised to keep clear of creeks and storm drains, and avoid walking, riding or driving through flood waters at all costs.
Authorities also recommend unplugging computers and appliances and avoiding landline phone calls for the duration of the storm.
