Sydney has posted its driest July in more than two decades but the city's parched parks and gardens have finally collected some much-needed rain.

The city's rain gauges gathered 12.6 millimetres as of the monthly cut-off of 9am on Monday, the least for a July since 1995, according to Bureau of Meteorology data for Observatory Hill.

A rain band had brought about 6 millimetres of rain by late afternoon, Sydney's first rain in 16 days, which will be put on the bureau's August ledger.

The arrival of another low pressure system on Thursday should bring the next chance of reasonable falls, Tom Hough, a meteorologist with Weatherzone, said. The bureau is forecasting 8-15 millimetres for the day.