“[The rollout targets] were always ambitious," he said on Friday at a lunch in Sydney held by the Australian Computer Society. "We wouldn't have been so aggressive if we'd known how tough it was for the company.

"So I think that that was an area where we were overly ambitious."

He added it was "undeniable" that NBN Co had failed to meet the rollout targets set for June 2013, or the targets of the first or second NBN Co corporate plan.

It was therefore "fair to say that the construction model could be legitimately criticised", he said.

Despite this, he said the project had been "well managed" if looked at as a whole and not just in terms of "how many leads have been put into homes over the last three years".