The security alert arose after a passenger inadvertently passed from an unscreened to a screened area at Sydney Airport, a Qantas spokeswoman said.
They were already inside the airport at the time.
The passenger then boarded flight QF487, which landed in Melbourne just after 9pm on Wednesday.
The Australian Federal Police was warned of potential security screening issues for the flight. After the plane arrived in Melbourne, officers helped Qantas security staff escort the 225 passengers and crew out to the baggage terminal.
"As a precaution, all passengers on QF487 were escorted from the aircraft when it landed in Melbourne and taken through the screened part of the airport into the unscreened area, thereby avoiding compromising the secure section of the Melbourne terminal, which would have required all passengers who had already been through security to be rescreened," the Qantas spokeswoman said.
"We will investigate to understand how this incident occurred and we apologise for any inconvenience to passengers on the flight."
The United Workers Union, which represents security officers, condemned the breach, pointing to it as the latest warning sign of serious shortcomings in Sydney Airport staffing.
The public should be alarmed that a person could unknowingly or otherwise avoid security screening measures, union property services coordinator Damien Davie said.
"Unfortunately what aviation security guards who work in screening say is that understaffing, a failure to adhere to set limits for workers examining baggage X-rays, a lack of training and other serious workload issues mean security is at risk of being compromised regularly," he said.
The security bungle did not cause severe delays, with journalist Patrick Durkin - who was on the flight - saying passengers were only delayed by about half an hour after they landed.
Mr Durkin said the plane's captain initially advised passengers to remain in their seats after landing, then announced their flight was deemed unscreened.
"(They) then said that we would, as a group, be escorted from the gate," Mr Durkin told AAP.
"They said that you're not to go to the toilet, you're not to remove anything from your person."
He said officers were holding machine guns as passengers were walked out.
Sydney Airport declined to comment on the security issue.
The airport on Thursday announced it would hold a second jobs fair to meet ongoing demand for staff, with security positions among more than 4000 vacancies heading into the September school holidays and Christmas.
The jobs fair will be held on September 21 and cover employers including the Australian Border Force, the Australian Federal Police, airlines and retailers.
"The aviation industry continues to be challenged by labour shortages, and with the passenger recovery tracking ahead of the workforce recovery, we're doing everything we can to support the 800 organisations at the airport recruit the right people for these vital roles across the airport," Sydney Airport chief executive Geoff Culbert said.
Jobs up for grabs would include aviation screening positions and biosecurity inspection officer roles, along with positions for baggage handlers, border force assistants and freight handlers.
