More tremors have been felt along the NSW Mid North coast overnight, after a magnitude-4.2 earthquake was recorded at 1:28pm off the coast of Coffs Harbour yesterday.

There is no tsunami threat from the earthquake.

A Geoscience Australia expert said he could not discount the possibility the series of earth tremors felt on the coast in recent days was not the lead-up to a larger earthquake.

Seismologist Phil Cummins said around 40 small quakes had been recorded between South West Rocks and Macksville since Friday.

The Geoscience Australia website recorded a magnitude-3.1 quake at Bellingen on Wednesday.

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Mr Cummins said the earthquake 'swarm' was a response to stress that slowly built up in the Earth's crust due to forces exerted at the plates.

"It's difficult to say what will happen in the future," Mr Cummins said.

"There is a pattern, a well-known pattern, of foreshocks — small earthquakes can build up to bigger ones.

"We don't really see that happening here. It's just a swarm where we will get dozens, possibly hundreds of them and they'll just eventually die out."

Residents phone in reports of tremors

The map shows the locations on the NSW Mid North Coast where earth tremors have been recorded in the last seven days. ( Supplied: Geoscience Australia )

Callers to the ABC Fiona Poole's Breakfast Program have reported feeling the unusual activity.

Penny from Congarinni North just near Macksville said she felt a bit of a bang at about 6:20am on Wednesday.

"A big boom and then a boom which we thought was above the house and then windows were gently rattling," she said.

"It was very weird and it gave us goosebumps.

"We heard the ones yesterday, so this morning when it boomed above the house we knew what it was straight away.

"I haven't felt any of them but heard them. I thought a possum was landing on the roof."

South West Rocks and Macksville reports

Margaret in South West Rocks said she had felt three tremors.

"I've been watching a lot of tennis and sitting in the chair and it just came like a push from behind."

Eric in Macksville said he first felt a tremor on Friday.

"We had a small tremor about 4:30pm. I was out doing some pruning and I thought it was someone doing some blasting near the bridge they're building," she said.

"On the 20th at 9:00pm I was watching the ABC News 24 and I thought it was a roll of thunder.

"Then on Sunday at 4:00am that one shook the house a little bit.

"At 9:00pm on January 22 there was a definite louder roll, a lot longer but it didn't shake anything.

"Then at 7.45am this morning there was the loudest bang. It was a real bang followed by a rumble for about five seconds."

Scotts Head resident reports rumbles

There were also reports from Scotts Head, about 90 kilometres south-east of Dorrigo and 50 kilometres north of South West Rocks.

Local resident Moira said she was glad to have the mystery cleared up.

"There were about 10 little rumbles on Saturday and a big one early on Sunday morning," she said.

"We thought it was thunder at first and we went and had a look and no, there's no clouds or thunderstorm.

"Some people said their houses shook a bit. We didn't feel it at our place but it was rumbling like thunder."