Australian airline Qantas (ASX: QAN) was forced to ground a plane after flight attendants discovered five rats inside medical equipment just minutes before passengers were due to board.

The baby rodents were found on a Boeing 767 during a pre-flight safety check at Sydney Airport. The plane was due to leave for Brisbane on Tuesday afternoon.

'We don't know how they got there. The aircraft was at the gate for some time before departure and we are investigating,' AFP quoted a Qantas spokesman as saying.

'It is a rare occurrence. We have no record of it ever happening before,' the spokesman said.

Despite the company's assertions, ABC reports Scott Connolly from the Transport Workers Union says the discovery is not unusual.

"Our members working out there report that they've increasingly reported concerns about hygiene, sanitation and this isn't the first occasion that they've reported rodents on aircraft and around their workspace," Mr Connolly said.

Engineers inspected the plane on Wednesday but found no damage to the wiring systems. The aircraft was due back in service on Thursday.