Mr Brown’s August diary mentions a "meeting briefing" with Serco on August 8.
When asked about the meeting, Mr Brown said, "Serco requested a meeting with me, which I accepted as I do with many stakeholders".
"No particular proposals were discussed".
"The Minister of Health must immediately and fully disclose the nature, purpose, and content of his discussions with Serco.
"A decision of this magnitude, which could fundamentally reshape public healthcare for a generation, cannot be conceived in secrecy and executed without public consent."
A spokeswoman for Mr Brown did not respond to further questions from the Otago Daily Times, including whether Serco was interested in involvement in the new Dunedin hospital project.
Health New Zealand Te Whatu Ora (HNZ) signed construction contracts with Australian giant CPB for the $1.88 billion hospital earlier this month, but recent mega-projects in the country, such as Transmission Gully, had been public-private partnerships.
A Serco spokeswoman said the meeting with Mr Brown was an introductory discussion with Serco’s Asia Pacific chief executive Andrew Head, who was in New Zealand at the time to support a charity event.
"The meeting provided an opportunity to share information about Serco’s work in New Zealand."
Serco is a multinational company supplying defence, health, space, justice, migration, customer and transport services.
In February 2011, Serco started operating the Mt Eden remand prison in Auckland.
The firm was heavily criticised for the existence of "fight clubs" within the prison that were not investigated until after their existence became public knowledge in July 2015, when footage emerged online.
This led to Serco's contract to run Mt Eden prison being revoked and its operation returned to the Department of Corrections.
Serco was ordered to pay $8 million to the New Zealand government as a result of problems at Mt Eden Prison.
Serco continues to run Auckland South Corrections Facility (Kohuora) and it also delivers healthcare services in Australia, the South Pacific and the United Kingdom.
"Before the nation is locked into a long-term contract with a company defined by its failures, the public has a right to know what is being planned in its name."
Green MP Francisco Hernandez said he was surprised Serco was even given an audience considering its reputation in New Zealand.
"The minister must be up front on why he's meeting a company with a long and sordid history.
"Given Serco’s track record of overcharging and under-delivery overseas, it would be deeply concerning if they were to be used as part of the hospital rebuild."
Mr Hernandez said the government’s lean towards the privatisation of services could mean Serco was a potential client.
"It's essentially even more concerning because Serco is anything but a standard private operator.
"I'm frankly astonished that they took the meeting in the first place. Even the appearance of meeting Serco as the Minister of Health really discredits him.
"We need to get to the bottom of what's actually happening. Maybe he's meeting them for a completely innocent reason, but given this government's track record, we highly doubt it."
