Mr Whittaker said details of his meeting with council resource management group manager Iain Maxwell were privileged and should not have been disclosed.
"We all should expect a level playing field when it comes to dealing with our local councils - trust and transparency are paramount," he said.
"HBRC issues consents, they monitor them and they prosecute and fine for breaches so it is therefore incredibly important they remain impartial. In our case it appears they have crossed that line and have acted incredibly unprofessionally."
The council's chief executive, Liz Lambert, said from time to time staff would inevitably have a personal interest in a regulatory matter the council was involved in.
"We have a standard approach to handle this type of situation and we take steps to ensure our regulatory role and other functions are undertaken without compromise," Mrs Lambert said.
"In this instance, while the staffer was ill-advised to use their work email address for the purpose of communicating information, that information was not privileged or confidential and was also available in the public domain."
That was disputed by Mr Whittaker who said: "If 'Joe Public' called up HBRC and asked for a briefing on a confidential meeting held between a company and the HBRC, would they have been supplied the information this staffer sent on? I don't think so."
Mrs Lambert said the email pre-dated any move by the council to take enforcement action against Te Mata Mushrooms.
It announced it was prosecuting the company on June 16.
"Since becoming aware of the potential for enforcement action against Te Mata Mushrooms, from March 2015, HBRC's resource management group has been very clear to maintain a tight control around information and who can access it," Mrs Lambert said.
A co-ordinator of the group Mr Hall sent the email to, lawyer Graeme Mansfield, said he believed Mr Hall had only passed on details that were in the public domain.
Mr Hall was entitled to his opinion as an individual and the group was set up to "support" Havelock North residents who wanted information on the mushroom farm, Mr Mansfield said.
Mr Hall, a regional geographical information system manager at the council, did not respond to a request for comment.
In a submission to Hastings District Council's district plan review last year he asked the council to take a role in alerting property buyers to the odour problem near the mushroom farm.
He said he only became aware of the problem after buying a property in the area.
As well as owning Te Mata Mushrooms, Mr Whittaker is involved in property development projects and is chairman of the Hastings City Business Association.