OTTAWA -- Six years after his explosive report on the F-35 derailed the Harper government's plan to buy the controversial stealth aircraft, federal auditor general Michael Ferguson is diving back into the fighter-jet file.

Ferguson's staff have been going over internal government records for several months, though the auditor general's office won't reveal exactly what aspects of the program are under the microscope.

The final report is scheduled for release in the fall.

Ferguson's last report on fighter jets in April 2012 was a bombshell which found senior defence officials twisted rules, downplayed problems and withheld information about the Harper government's plan to buy 65 F-35s.

The report forced the government to suspend the project pending a complete review, which eventually pegged the full cost of buying and operating the F-35s at more than $45 billion.

Six years later, Canada still has not chosen a new fighter jet to replace its aging CF-18s.

It is unlikely the auditor general will find the kind of serious problems with the government's efforts to buy fighter jets as the last time, as the overall procurement system has been revamped with a variety of checks and balances.

The new review could instead turn on the government's plan to buy interim fighter jets and its arguably slow progress in holding a competition to replace the CF-18s, said defence analyst David Perry of the Canadian Global Affairs Institute.

"How much benefit and at what cost will Canada obtain fighter aircraft by virtue of going through an interim fighter purchase as well as doing a life extension?" Perry said.

"For me, one of the biggest concerns is that the process from this point forward is not exactly lightning quick."

The Liberals announced in November that they plan to buy used fighter jets from Australia -- rather than new Super Hornets from Boeing -- as a stop-gap until the entire CF-18 fleet can be replaced.

The new planes were originally expected by 2025, but documents provided to industry last week show the new target is 2031, as the government plans to take its time with a competition.

The delay is almost certain to mean another round of expensive and complicated life extensions to the CF-18s, which will be 50 years old by the time the last are retired.

Previous reports have determined that any life extensions to the CF-18s beyond 2025 would be "a high-risk option in terms of cost, schedule and technical factors."

Many defence analysts and retired military officials have questioned the need for interim fighters and urged an immediate competition, rather than waiting several years.