A small earthquake has been detected off the NSW Central Coast, with residents reporting feeling the ground shake from Sydney to Newcastle.

Geoscience Australia said the magnitude 4 earthquake occurred at 10:02am on Tuesday, about 100 kilometres east of Gosford. The earthquake was initially measured at magnitude 4.1, but was later downgraded.

The Joint Australian Tsunami Warning Centre said there was no tsunami threat to the Australian mainland, islands or territories from the earthquake.

Marco Maldoni, a duty seismologist at Geoscience Australia, said the organisation had received up to 100 reports from people who felt the earthquake.

"There was no damage from an earthquake this size, but we received lots of reports," he said.