Police Minister Anne Tolley asked for the Independent Police Conduct Authority to investigate the police's handling of the case.
Labour police spokeswoman Jacinda Ardern pointed to a drop in public confidence in the police since the Roast Busters case, which a survey showed had fallen from 82 per cent to 76 per cent.
Mr Marshall defended this record, saying that the survey often fluctuated, had a margin of error of 3 per cent, and was roughly the same as this time last year.
The commissioner, who will soon finish his tenure, said the case was not a blot on his legacy because crime and road deaths had dropped steadily under his watch.
"I'm very bullish about what police have achieved. I think the New Zealand public is well served by this place."