Winter's grip tightened on Sydney and much of NSW, plunging some regions to record lows, with another bout of chill on the way.

Sydney's overnight low of 5.1 degrees at Observatory Hill on Sunday was the lowest July temperature since 2007. The most recent lower overnight reading was August 5, 2015, the Bureau of Meteorology said.

Camden was colder still, with -3.8 degrees at its minimum, and Richmond with -4.3 degrees. For both sites, it was the lowest overnight temperature since June 2010.

Inland regions broke records, such as Bathurst's -8.1, a new low for any month.

The defence facility at Marrangaroo near Lithgow was the coldest spot in the state with a regular gauge, dropping to -11.1.