One of New Zealand's top comedians has been criticised by a health expert for his anti-vaccination comments.

Flight of the Conchords star Jemaine Clement waded into Jim Carrey's anti-vaccination Twitter rant on Wednesday, sending more than 20 tweets defending the actor's "debate" on the controversial topic.

Immunisation Advisory Centre director and senior lecturer of general practice and primary health care, Dr Helen Petousis-Harris, described Clement's tweets as "a load of nonsense".

A load of nonsense that was potentially damaging, given his public profile.

READ MORE: Jim Carrey goes on anti-vax rant

"Rumour can be damaging," Petousis-Harris said. "It can always have an impact on some people who suddenly become concerned about something that is actually untrue."

Carrey, who has previously dated anti-vaccination activist Jenny McCarthy, posted a series of angry messages to his 14 million followers accusing the Californian government of being a "corporate fascist".

The 53-year-old later back-pedalled and said he was not anti-vaccination but anti-neurotoxins (citing thiomersal).

Clement also said he was "not promoting anti-vaccination" but "defending investigation".

He went on to say "pressure groups are important" in the right to demand regulation.

In response to Clement's argument that people should continue to question the safety of vaccines, Petousis-Harris said it seemed Clement was implying that was not the case.

"It's always right to keep assessing the safety of vaccines, which we do. He's making a straw man argument.

"He's suggesting that doesn't happen, but it happens all the time, globally, on a huge scale."

New Zealand used to have some of the worst immunisation coverage in the OECD, however it now sits around 94 per cent.

WHAT IS THIOMERSAL?

The ingredient Carrey mentions in his tweets, thiomersal (which he spells thimerosol), is a mercury-based preservative used in some vaccines, as well as nasal products and tattoo inks.

In the United States, thiomersal has been removed from or reduced to trace amounts in all vaccines routinely recommended for children younger than 6 years, with the exception of inactivated influenza vaccine.

National Immunisation Schedule vaccines used in New Zealand, including MMR, have not contained thiomersal for many years.

It was removed not because there was any evidence of risk, but because removing mercury from any source, if you can, is always a good idea.

Source: Immunisation Advisory Centre.

INTERNATIONAL EXPERTS BITE BACK

International experts have also been quick to disagree with Carrey.

"Yes, Mr. Carrey, you are anti-vax," science writer Phil Plait says, accusing Carrey of cherry-picking information to make his case. "You can't pick and choose what to believe in and what not to, and then say you're not anti-vaccine."

Professor Arthur Caplan, of New York University's School of Public Health, agrees, accusing Carrey of spreading misinformation.

"The childhood vaccine schedule has been proven safe, and arguments about 'dangerous' ingredients have been proven incorrect over and over," he writes.

"Not only is Carrey ignorant when it comes to vaccines, he is a fool when it comes to using terms like fascism.

"Fascism is when a government imposes its will upon the people by fiat. Nothing like that has happened in California – not even close. The legislature heard testimony, debated and then voted through the law to end liberal exemptions. Passing a law through the legislature and having the governor sign it is called democracy!"

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