The revelation came as authorities in both New South Wales and the Australian Capital Territory made efforts to prioritise medically mandated testing at swamped testing centres.
“SydPath last night incorrectly messaged more than 400 people, advising them they had tested negative to COVID,” SydPath said in a statement.
“As soon we became aware of the issue this morning, SydPath immediately commenced a process to contact impacted people.
“An emergency response team is now investigating the cause of this mistake, which is believed to be human error.
“We sincerely apologise to all those impacted.”
SydPath medical director Professor Anthony Dodds read out a statement to the media on Sunday afternoon, blaming the “very large volume of tests that we’ve been contending with”.
NSW Premier Dominic Perrottet urged residents to consider using a rapid antigen test (RAT) if they weren’t legally or medically required to get a PCR test.
“I know there are long queues right across our state, so if you do not feel unwell there is no need to receive a PCR test. You are taking a spot in the queue from somebody who needs a test,” he said.
“We just don’t believe that the requirement of a PCR test for somebody who is not unwell is actually supportive of that view.
Authorities in the ACT are also struggling to keep up with testing demand.
On the Sunday night, ACT Health announced testing centres at Garran and Mitchell would be testing only close contacts, people with COVID-19 symptoms and returned international travellers from 11am to 3pm on Monday.
Testing for other reasons is still available outside of those hours and at the Kambah testing centre.
“In the context of current testing pressures, for fully vaccinated close contacts who require a test to exit quarantine, a negative test result from a test collected on or after day 5 will be considered sufficient to exit quarantine after day 7 provided they are symptom-free,” ACT Health said.
“If they are notified they are a close contact on day 4 after their exposure, and are fully vaccinated and symptom-free, they may wish to delay their initial test to day 5 or later.”
In the 24 hours to 8pm on Saturday NSW recorded a new daily high of 6394 COVID-19 cases from more than 109,500 tests.
ACT Health on Sunday reported 71 new cases.
Professor Dodds said none of the sailors involved in the Sydney to Hobart yacht race were believed to be caught up in the cohort of false negatives.