More than 400 people were incorrectly told they had tested negative to COVID-19 but were positive for the virus, a Sydney hospital said.
A statement from St Vincent's Hospital said SydPath became aware of the issue on Sunday morning and immediately started a process to contact affected people.
Professor Anthony Dodds, the medical director of SydPath, told reporters: "Owing to the very large volume of tests that we have been contending with at SydPath, last night we incorrectly messaged 400 patients who had been swabbed at our centres from 22 December to 23 December advising them that they had tested negative to COVID.
"These people had, in fact, tested positive to COVID.
"As soon as we became aware of this issue this morning, SydPath immediately commenced a process to contact these impacted individuals."
The incorrect messages were sent out on the evening of Christmas Day, and the hospital has apologised to all those impacted.
In a statement, it added: "An emergency response team is now investigating the cause of this mistake, which is believed to be human error."
A message received telling a patient to disregard their negative COVID-19 test. Source: SBS News
A second message was received at 10.33am on Boxing Day saying the COVID-19 test was positive and that the case had to immediately self-isolate.
A third message said: "Please disregard previous text message regarding negative Covid test.
"Your sample was retested due to a laboratory error and is COVID-19 POSITIVE.
"For further information please contact the laboratory."
It comes as on Sunday as shoppers took to the streets for Boxing Day sales across Australia.
NSW Health Minister Brad Hazzard said NSW is now reporting up to 130,000 to 160,000 tests a day.
"On the face of it, [that] sounds good but then we need to consider why are people being tested and why are we putting the stressors on our pathology system?," he told reporters on Sunday.
