A man, who tested positive for coronavirus after towing his caravan from Victoria to a Sutherland Shire caravan park, entered NSW on July 7, a day before the border was closed.
NSW Chief Health Officer Kerry Chant said the man "has reported minimal contact with anyone in NSW apart from his partner and two friends".
"The man subsequently tested positive and is in hotel isolation and the three contacts are in quarantine," she said. "His partner has tested negative."
A man, who towed his caravan from Melbourne to a Sutherland Shire caravan park in the last few days, has tested positive for coronavirus.
Health Minister Brad Hazzard described the case as "troubling".
"This is a man in his 20s who has tested positive in the Sutherland area," Mr Hazzard said at a media conference on Friday afternoon.
"He is from Melbourne and he has come up in the last few days.
"He has tested positive. We are doing investigations at the moment of contacts etc and that will continue.
"At this point because it is so early, I won't be saying much more.
"He did tow his caravan up from Victoria - quite a long drive, I understand 14 hours straight through, I am told.
"I thank him for coming forward - that is excellent that he came forward, and I encourage others in a similar situation that may have come from Victoria or Melbourne particularly in the last few days to make sure you are on high alert, because we in NSW are on high alert.
"We want you to get tested, and it is crucial you do, because otherwise, you will be the instrument of seeding or one of the possible instruments of seeding here in New South Wales."
Mr Hazzard said he would not name the caravan park at this stage while NSW Health carried out tracing.
"NSW Health is looking at what other measures may be necessary for others in and around that caravan park, but obviously first of all there needs to be full questioning to know what his contacts might have been."
Mr Hazzard said people who had come from Melbourne recently would know they were a risk to people in NSW.
"I would ask you to consider packing up your caravan, your tent, or whatever else you've got and head home to Victoria," he said.
NSW Chief Health Officer Kerry Chant said the man was not aware he had coronavirus when he crossed the border into NSW.
"He was travelling in a car and in a caravan and did the trip quite swiftly," she said. "We understand that he has had minimal exposure [to others] on his route up here," she said.
Dr Chant said there was one other person with the man in the caravan and there were another two other possible contacts.
In another new Sydney case, the Crossroads Hotel at Casula has been forced to close after a woman and man, who attended the pub on Saturday night, tested positive for COVID-19.
The two cases came to the attention of health officials after results for the last 24-hour testing period were released earlier on Friday.