The latest case, in which a prisoner suffered injuries, prompted Corrections chief executive Ray Smith to take "firmer steps" to address Serco's problems.
"I have learned today that another prisoner transported this week from MECF to one of our other prisons has arrived there with injuries and he has made serious allegations about his treatment at MECF," Mr Smith said last night.
In a cynical move, the Corrections Department reported the case at 6.03pm, after the television news bulletins had begun.
It was hugely embarrassing timing for Mr Lotu-Iiga, coming hours after he hauled in two top-ranking Serco executives and told media that there were no further cases of mistreatment at the prison.
Before the meeting, he had warned there would be "serious consequences" if Serco failed to come clean about any undisclosed events at the jail, where the UK-based company has a $300 million management contract.
A spokeswoman for Mr Lotu-Iiga confirmed last night that the minister was not told about the latest incident by Serco's executives during their two-hour meeting at the Beehive.
Mr Smith did not attend the meeting with the Serco bosses.
After learning of the latest case, Mr Smith said he said was taking legal advice and "considering the full range of options available to me in our contract with Serco who run this facility".
Labour and the Greens renewed their calls for Serco's contract to be cancelled.
The latest incident had echoes of Nick Evans, who was transferred to Ngawha Prison with injuries last month and later died at Whangarei Hospital.
The Coroner is investigating his death.
On top of its $300 million, 10-year contract at Mt Eden, Serco is entitled to up to $1.2 million a year in performance-related fees.
These fees were reduced if its contract was breached, including a $150,000 fine for the deaths of prisoners from unnatural causes.
Earlier in the day, Serco managing director Paul Mahoney said the company was taking the allegations of mistreatment seriously.
He said prison management had put the prison into lockdown this week for a full search.
Before the latest incident, Prime Minister John Key said he still had confidence in Mr Lotu-Iiga and there was a need to wait until all the facts were known because various "vested interests" were making claims for their own purposes.
He was reluctant to say Mr Lotu-Iiga had been let down by his department but said there was information that wasn't handed on.
"So in that regard, yes he has been let down. But I suspect Serco have had information ... they haven't shared with the ministry as quickly as they should've."
- By Isaac Davison of the New Zealand Herald
