In its letter to the ministers requesting the extension, lawyers for the board said it was dealing with more than 28,000 pages of material including the council's 5000-page application, more than 5500 of submissions from 384 submitters, nearly 7000 pages of evidence and more than 6000 pages of other material.
The transcript of proceedings over the 29 days of hearings ran to more than 3700 pages.
In its letter the board's lawyers said: "the hearing itself involved highly contested technical expert evidence on a number of key matters requiring careful and methodical consideration and determination by the board".
The board, chaired by retired High Court judge Lester Chisholm, had been working extended hours and over weekends during the hearings and it considered an extension was required to ensure it delivered a "robust and accurate decision".
A spokesman for Ms Adams said she and Dr Smith decided to approve the extension on Monday and notified the board yesterday.
The board now expects to produce a draft report by April 15 which will be open to submissions until May 16. It will produce a final report by May 28.
Regional councillors will then vote on whether to proceed with the scheme and are also required to carry out public consultation on the proposal. The council is considering investing up to $80m in the project, with central government and other investors also contributing to the cost of the scheme.
The project is being driven by the council's commercial arm, Hawke's Bay Regional Investment Company (HBRIC). Chairman Andy Pearce said yesterday the company supported the extension because "a high quality and robust decision is in the best interests of all parties".
HBRIC has been working to a tight deadline to get construction of the scheme under way this year.
Dr Pearce said the new timeframe would make the process "slightly more complicated during the council's decision-making and public consultation process" but it was important to get a quality decision.
"In the journey of these sorts of things you have to cope with the odd hiccup and this is just one of those."
He welcomed Ms Adams and Dr Smith's quick decision.
"That's another piece of uncertainty out of the process.
"The commissioners now know they've got the right amount of time."