The father of New Zealander Sarah Carter who died mysteriously in Thailand refuses to accept the Thai authorities' assertion that she probably did not die as a result of exposure to insect control chemical chlorpyrifos.

"They've basically just shown that they'd prefer to cover it up rather than to put it out it in the open," Richard Carter told NZPA.

Ms Carter, 23, fell violently ill on February 3 while staying at the Downtown Inn in Chiang Mai and died a day later.

Her two New Zealand friends and travelling companions, Emma Langlands and Amanda Eliason, also fell ill but later recovered.

An investigation by TV3's 60 Minutes which aired on Monday found trace elements of the chemical chlorpyrifos, used to kill bed bugs, in the hotel room Ms Carter was staying in, prompting calls for Thai authorities to investigate.

Chiang Mai Public Health Office deputy chief Surasing Visaruthrat, who yesterday chaired a meeting attended by doctors and experts from various agencies, said the chlorpyrifos theory "carries little weight", the Bangkok Post reported.

"We can't jump to a conclusion that toxin exposure was the cause of the death because the substance was allegedly found in the hotel room alone, not in the victim's body."

An elderly British couple, a Thai tourist guide and a Canadian man also died after staying at the Downtown Inn or using its facilities, while two other women died in similar circumstances within one month.

Dr Surasing said experts would meet again after receiving test results of samples collected from the victims' bodies, which were sent for testing at laboratories in the United States and Japan.

Mr Carter said the findings did not surprise him at all.

"They did absolutely nothing for over three months and most of the things that they have done have lacked any thoroughness or integrity.

"I wouldn't place a lot of faith in the Thai health authorities."

Mr Carter said he would not be satisfied until the World Health Organisation had conducted a full, independent investigation into the deaths.

Meanwhile, the Green Party said chlorpyrifos was scheduled for "priority reassessment" in New Zealand in the Environmental and Risk Management Authority's (ERMA) briefing to the incoming minister in 2008, yet it is still in widespread use.

"ERMA has had chlorpyrifos on the reassessment list for years, but so far nothing appears to have happened," spokeswoman Catherine Delahunty said.

"Given its widespread use in New Zealand, even in stone fruits and wheat, they must undertake the urgent reassessment that should have already happened."

A 2009 study found residue of chlorpyrifos in food, including biscuits, bread, muesli, bran flakes and kiwifruit, she said.

"We want safe, healthy food for everyone. We don't want to wait for someone to die before we stop using acutely toxic chemicals."

Fertiliser supplier Ravensdown, which sells an insecticide containing chlorpyrifos, yesterday told NZPA extensive research had been carried out into chlorpyrifos use, which showed it to be safe.

"We provide extensive information on the correct use of products and take all available measures to ensure our products meet all regulatory requirements. We aren't aware of any adverse health and safety effects arising from the use of this product," spokesman Mike Whitty said.

Furthermore, users of products containing chlorpyrifos in New Zealand must undergo training and obtain an approved handlers certificate before being sold the products, he said.

Mr Carter said although it was somewhat disturbing that the chemical was being sold here, he trusted that New Zealand had far higher health and safety standards than Thailand.