Health authorities in NSW are scrambling to identify the source of three mystery coronavirus infections, in an attempt to link them to a growing cluster in Sydney’s south west.

Forty confirmed COVID-19 cases had been linked to the Crossroads Hotel outbreak in Casula, an increase on Wednesday’s number of 34.

Two of the three mystery infections are in Sydney’s south-western suburbs while the third is from the Wollongong area.

Several businesses in a swathe of Sydney suburbs have been closed for cleaning after infected people visited them in the past two weeks.

Chief medical officer Kerry Chant said the fact the state’s coronavirus detectives had not been able to trace three infections was an issue.

“Obviously, we are concerned, when we find cases that can’t be linked back, because it does indicate we have missed a chain,” she said.

“So we’ll be re-interviewing cases and trying to ascertain any contact points and updating the community about additional actions.

“But it does highlight the fact that at this critical point, we do need people to continue to practise social distancing, make sure you isolate yourself if you have the most minimum of symptoms and get a test.”

There are now 40 confirmed infections linked to the Crossroads Hotel in Casula.

At least four new cases have been linked to the hotel outbreak, including one person who contracted the virus when they attended the Hurricanes Grill in Brighton-le-Sands with a known case.

A Victorian who flew from Melbourne to Ballina, in northern NSW, on July 12 has also been diagnosed with coronavirus.

“The individual was screened on arriving in Ballina airport and since arriving, they were in 14-day self-isolation and any potential close contacts are being followed up,” Dr Chant said.

More than 22,800 tests were conducted in NSW in the 24 hours to 8pm Wednesday, and 3400 of those were in south west Sydney.

On Wednesday, Magpie Sports Club in Croydon Park, in Sydney’s inner west, closed for cleaning after a patron had tested positive for COVID-19.

“The patron visited Magpie Sports Club on Friday 10th of July between 8pm and midnight,” the club said in a statement on Facebook.

Several states have announced tougher restrictions on travellers from NSW amid the growing cluster in Sydney’s south west.

Queensland barred anyone who has been in the Western Sydney suburbs of Campbelltown and Liverpool from entering the sunshine state, just days after reopening its border to people from NSW.

The Northern Territory and South Australia have also delayed plans to allow people from Australia’s most populous state in to their jurisdictions.