Five people from Melbourne coronavirus hot spots have been detected during a new screening initiative at Sydney Airport.
Health officials also found a woman, who arrived with symptoms at Central station, had been tested in Melbourne but boarded an XPT before getting the results.
"That's about as silly as it gets," Health Minister Brad Hazzard said at a news conference on Thursday.
Mr Hazzard and NSW Chief Health Officer Kerry Chant also gave details of a man, who arrived in Victoria from overseas and tested positive for COVID-19 while in hotel quarantine in Melbourne.
After 14 days in quarantine, he was allowed to leave the hotel without a further test and flew to Sydney where he worked for two days at Woolworths Balmain.
After displaying symptoms, he underwent another test, which was positive.
The man's co-workers were directed to self-isolate and NSW Health is carrying out contact tracing.
The man travelled to Sydney on Jetstar Flight JQ 510 on June 26.
There were eight new cases of COVID-19 in NSW in the 24 hours up until 8pm on Wednesday, all returned travellers, who are in hotel quarantine.
A total of 18,347 tests were notified in the period, compared with 16,243 tests in the previous 24 hours.
Mr Hazzard said the number of people presenting for testing was "amazing".
"That's very much while the NSW is doing so well," he said.
Mr Hazzard signed Public Health Order on Wednesday evening, which came into effect at midnight, giving police the power to fine people who enter NSW from Victoria's designated coronavirus hot spots.
Mr Hazzard said screening, which began this morning at Sydney Airport and Central station had been "quite good".
Some people who had come from hot spots were on their way to Queensland and Northern Territory.
Mr Hazzard said he personally phoned the ministers in the state and territory the passengers were on their way.
Mr Hazzard has asked Victorian authorities to reciprocate by screening passengers heading for NSW.