The cracks - which Qantas said do not immediately compromise safety - are a worldwide issue affecting Boeing's popular 737 NG model aircraft and appear on the plane's "pickle fork", which attaches the wings to the body.

The US Federal Aviation Authority on October 2 ordered inspections within seven days of all 737 NGs that had operated more than 30,000 flights. Planes with more than 22,600 flight cycles needed to be inspected within the next 1000 flights, which for Qantas' equates to about seven months' operation.

However, Qantas said on Wednesday night that it would speed up that time-frame after it discovered one jet with cracks, which had flown just under 27,000 missions. The airline has grounded that plane for repairs.

A Qantas spokesman said that out of "an abundance of caution" the airline would inspect the remaining 33 jets with more than 22,6000 cycles in its fleet by the end of this week, rather than over the next seven months.

"Qantas would never operate an aircraft unless it was completely safe to do so," the spokesman said.

"Detailed analysis by Boeing shows that even where this crack is present, it does not immediately compromise the safety of the aircraft - as indicated by the time-frame given by regulators to perform the checks."

Qantas has 75 Boeing 737s in its fleet, which operate the majority of its domestic flights.

The cracking issue with Boeing 737 NGs is unrelated to the worldwide grounding of the jet's updated model, the 737 MAX, following two deadly crashes.

Virgin Australia has checked all 19 aircraft in its fleet meeting the threshold for inspection and did not find any issues, a spokeswoman said.