The accident occurred around 11.30pm on Sunday near the town of Greta, about 180km northwest of Sydney, in an area famous for its vineyards and wedding spots.

“I understand they had been at a wedding together, it’s my understanding they were travelling together … presumably for their accommodation,” NSW police acting assistant commissioner Tracy Chapman said during a televised media briefing.

At this stage it appeared to be a single vehicle collision, Chapman said, as police tried to identify all passengers. Police said some people could be trapped beneath the bus, which was lying on its side.

The driver of the bus, a 58-year-old man, is under arrest and expected to be charged over the accident. He has been taken to hospital for mandatory testing.

Heavy fog was present in the area when the accident occurred but Chapman said the cause of the accident had not been determined.

Prime Minister Anthony Albanese expressed “deepest sympathies” to the families of the people killed and injured.