New Zealand troops returning from reconstruction duties in Afghanistan face delays of up to six days after confirmation of engine damage to an Air Force Boeing 757.

Engineers at the Whenuapai air base near Auckland determined yesterday that the Rolls-Royce engine, on which a compressor stalled 600m above North Shore City on Thursday, would have to be replaced.

This will take at least a week, meaning the Defence Force will send 125 troops back to New Zealand on civilian flights. They are all expected back by Monday, August 8.

A similar replacement job in Fiji last year on the Air Force's other 12-year-old Boeing 757 cost almost $4.9 million.

That aircraft is undergoing routine maintenance in the United States, but its replaced engine has since been refurbished in Hong Kong and will now be attached to the Boeing hit by the latest mishap.

The troops were supposed to have been collected this weekend from an undisclosed country neighbouring Afghanistan, to arrive back here on Tuesday.