This week, however, the Whanganui District Health Board (WDHB) wrote to Dr Cresswell expressing concern at his "recent views" as reported in the Chronicle, ASMS executive director Ian Powell said.
The DHB was worried his views could "damage the reputation of the Whanganui District Health Board", he said.
"It's outrageous that this health board apparently considers it's unacceptable for doctors to express an opinion on matters affecting patients and their ability to deliver the best possible health care to New Zealanders," Mr Powell said.
"Memo to Whanganui DHB: doctors have a right and in fact a responsibility to speak out publicly on these matters without you trying to shut down the debate. Pull your head in."
Mr Powell said the letter asked Dr Cresswell to attend a meeting to discuss the concerns, and the letter cited various documents including the DHB's code of conduct and State Services Commission Standards of Integrity and Conduct.
"Unfortunately, the letter omitted the most relevant document of all, the DHB collective employment agreement signed by all of the DHBs," Mr Powell said.
"This binding employment agreement includes specific clauses to protect the right of doctors to engage in public debate on matters relevant to their expertise and experience."
Mr Powell said the ASMS would attend the DHB meeting with Dr Cresswell.
"Dr Cresswell is not doing anything out of the ordinary in expressing his concerns about the TPPA, and we will tell the DHB so in no uncertain terms. His views are in the mainstream of the medical profession.
"Doctors throughout the country have expressed concern about the impact of the TPPA and the Government's secrecy surrounding its negotiations."
He said the ASMS was unaware of any other DHB "trying to gag senior doctors in the TPPA debate".
"Whanganui DHB is the outlier, and if there's any harm being done to the DHB's reputation, then it's self-inflicted. Unfortunately, there is too much authoritarianism in the leadership DNA of this DHB."
Following last year's annual conference, the ASMS voted to oppose the TPPA "on the grounds that health care will suffer from the loss of national autonomy that may result".
Dr Cresswell said he was unable to comment.
Asked for comment on the ASMS statement, WDHB chief medical officer Frank Rawlinson said he was "very disappointed" that the ASMS had seen fit to release details of a planned meeting between himself and one of the DHB's senior doctors before the meeting had taken place and therefore before any outcome had been determined.
"As I stated in my letter requesting that he meet with me to discuss his recent actions, I respect his rights under the NZ Bill of Rights Act 1990 (Freedom of Expression and Freedom of Association). I do, however, reserve the right to discuss with Dr Cresswell the effects that his very public behaviour may have on the DHB," Dr Rawlinson said. "I have no intention of challenging his views but rather the way in which he chose to express them."
