Health authorities knew a now-scrapped coronavirus vaccine risked participants falsely testing positive for HIV, but they didn't expect it to become a problem.
The University of Queensland and biotechnology company CSL abandoned trials of its COVID-19 vaccine candidate after one of the ingredients used - a protein found in HIV - caused 168 participants to return false positive tests to human immunodeficiency virus.
They were all the participants who didn't receive a placebo.
The use of the protein caused participants to create antibodies, and they were told it could show up in tests.
The government has torn up a deal to get 50 million doses.
Department of Health boss Brendan Murphy says officials knew about the false positive risk but thought it would be "very, very" unlikely to occur because the fragment of the HIV molecule was tiny.
"It was unfortunately an unexpectedly high rate of false positives that resulted when the data came in," Professor Murphy said.
"Unfortunately, it just became a bigger problem than anyone had anticipated."
Prof Murphy said the candidate still looked promising but the risk to public confidence was too great.
"We can't have any issues with confidence," he said.
An extra 20 million doses of the AstraZeneca vaccine and a further 11 million of Novavax have been ordered.
Scott Morrison has reassured Australians the vaccine rollout is still on track to begin in March next year.
The government spent $1.7 billion towards the AstraZeneca jab and University of Queensland/CSL version.
The prime minister defended the spending despite the CSL result, saying every cent spent on the vaccine race was worth it.
"The expectation there would be a 100 per cent success rate across these is naive. It's just not true."
CSL has assured participants they did not contract HIV.
"There is no possibility the vaccine causes infection," the company said in a statement.
Mr Morrison reassured the public any vaccine would have to be given the tick of approval by the Therapeutic Goods Administration
"Without the tick there's no jab," he said.
"The truth is, we're on track. The system's working as it should and Australians are protected, as always."
Prof Murphy said the rollout could even be quicker because more AstraZeneca vaccines had been ordered and it's already in production.
The government was told on Monday about the trial results, with cabinet agreeing on Thursday to cancel the deal based on advice from health experts.
Labor's health spokesman Chris Bowen said the government had made the right call, but wants more possible vaccines to be in their net.
"These deals are the key to ensuring that Australian's can return to a more normal life next year, and our economy can come back to a more normal situation next year," he told reporters in Sydney.
Britain started its coronavirus vaccine program this week, with authorities saying there have been two reports of anaphylaxis and one of a possible allergic reaction since the rollout began.
It has prompted Britain's medicine regulator to tell anyone with a history of anaphylaxis to a medicine or food to not get the Pfizer-BioNTech vaccine.
Australian Associated Press