Ms Carter died of myocarditis - acute heart inflammation - but tests are still being carried out in the United States and Japan to determine its cause.
Two other women have died in the city in similar circumstances, as well as a Canadian man who used the Downtown Inn's facilities.
Thai authorities have not been able to find any links between the deaths and the Downtown Inn, which is owned by the city's former mayor, remains open.
"I've heard nothing substantial came out of the investigation into the room. There was just a cursory sweep-through by the police and they okayed it," Mr Carter says.
"Having the ex-mayor as the owner of that building - when you're combining that with a known element of corruption in that country, then putting one and one together ..," he says.
Mr Carter would like a list compiled of all the deaths in the Downtown Inn over the past year "because I feel they weren't forthcoming with these other deaths, they were only uncovered through the media".
Thai authorities, under increasing international diplomatic and media scrutiny, have enlisted the World Health Organisation as well as the highly respected United States Centres for Disease Control to help determine what caused the deaths.
Thailand's Department of Disease Control, which has been involved in "active surveillance", said there had been no new cases of myocarditis or unexplained deaths since February 19.
Because deaths that occur outside of a hospital or in unexplained circumstances were the responsibility of the police, parallel forensic and public health investigations had been running.
"But the full integration of all such efforts is now in progress."
Mr Carter accepts that the laboratory testing is now in safe hands, but says statements from Thai officials made him question the thoroughness of other aspects of the investigation.
In a report to New Zealand's Ministry of Foreign Affairs and Trade, subsequently passed on to Mr Carter, the Governor of Chiang Mai said conferences had been held to inform media that the four deaths at the Downtown Inn were "coincidence".
"That just reeks of cover-up, to say that. How could you possibly call that a coincidence when there were four people who died in adjoining rooms?"
He said that in another report Thai authorities claimed most of the family of the British couple found dead in the Downtown Inn were satisfied with an autopsy pointing towards heart failure/disease.
That was despite the couple's son Stephen Everitt telling the Herald the findings were unbelievable: "They want me to believe they both had heart attacks at the same time. It doesn't make sense."
But the final straw came when Mr Carter learned of two young women dying after staying in adjoining rooms in a hotel on Thailand's Koh Phi Phi in 2009.
Food poisoning was initially suspected but later ruled out and the cause of the deaths remains unknown despite relatives complaining about the investigation.
"When I discovered that spate in 2009 the thought crossed my mind that had it been properly investigated ... Sarah may still be with us.
"And that gives me a little bit of pain but it helps drive me to get an answer."
He says his family are pulling together to cope with the grief and appreciate the support from work and school colleagues.
"It's not that pleasant but we've got to carry on as best we can. It's not like we're the only ones in the world suffering from lost children or siblings ... it's nothing particular to us, that's for sure."
Before she left on her trip, Sarah compiled a thorough list of things her family wanted her to bring home as gifts.
When her mother and brother collected her possessions in Thailand they found the list with only about a third of the things crossed off.
"Anna and Ryan went around and just bought a number of things that were on that list that she didn't get to do," Mr Carter says, his voice cracking.
"Sorry. It's just talking about those more emotional things ... it's hard to do. Especially her personality. Because she was, she was just marvellous."
