Australia's air safety body is to broaden its investigation into Qantas after the airline grounded six of its aircraft because of irregularities in maintenance records.

Peter Gibson, a spokesman for Australia's Civil Aviation Safety Authority, said on Wednesday that the agency would try to determine the extent of the maintenance record problem.

This comes just a month after a Qantas airliner made an emergency landing in Manila after an explosion - believed to have been caused by an exploding oxygen bottle - tore a large hole in the fuselage.

On July 28, a Qantas 737-800 returned to Adelaide after a landing gear door failed to retract and earlier this month a Boeing 767 bound for Manila turned back to Sydney after developing a leak of hydraulic fluid.

Qantas said the six B737-400s would be removed from service while the airline cross-checked maintenance records relating to work carried out at one of its Australian facilities.

The company's senior engineering manager said it was a record-keeping issue and there were no safety implications.

"What we picked up is a step in the maintenance check which does not look like it has been completed, and that's why we've got to go back and verify that," David Cox said.

"Until we are absolutely certain that everything is 100 per cent correct we cannot operate the aircraft," he told the Australian Broadcasting Corporation.

Three flights were cancelled on Tuesday night.