A Qantas pilot came close to having the plane's tail hit the runway at Canberra Airport after staff over-estimated the weight of a large group of primary schoolchildren by several tonnes.

The passenger jet became "nose heavy" during take-off on May 9 because the 87 schoolchildren had been mistakenly classified as adults when their tickets were booked, the Canberra Times reports.

That error meant the weight of the total 150 passengers was overstated by 3.5 to 5 tonnes.

A report by the Australian Transport Safety Bureau says the standard weight for an adult is 87kg while a child is 32kg.

The poorly distributed weight forced the pilot of the Boeing 737 to pull back on his elevator controls during lift off to ensure its tail did not strike the runway.