The location of screenings of Vaxxed across the country, including Whangarei and possibly Kaitaia, have been kept secret, WavesNZ notifying ticket holders of venues only a few hours in advance.
"I would really like to know what it is they need to hide," Dr O'Sullivan said, adding that WavesNZ, which described itself as a charitable organisation, was not a registered charity.
"Is it the fact that the director of the film [Andrew Wakefield, who published 'fraudulent' research linking the measles, mumps and rubella (MMR) vaccine to autism and bowel disease] is a discredited scientist?"
Wakefield's 1998 Lancet research had been described by Dr Dennis K Flaherty, of the University of Charleston, as having created a public health crisis. Dr Flaherty called his publication "the most damaging medical hoax of the last 100 years".
Dr O'Sullivan said immunisation worked at a population level thanks to herd immunity, because when the vast majority of the population were immune, a disease struggled to find new hosts to infect.
That meant babies and young children who had not yet been immunised were protected too, that level of protection becoming weaker if there were lower immunisation rates amongst older children and adults.
"Last year's measles outbreak in the Waikato, where 71 of the 89 [victims] had not been vaccinated, is a good example of how a preventable disease can take hold in a community," he said, calling on WavesNZ to "come out of the closet" and allow parents to make an informed decision.
"All children in our community deserve to be protected by a best practice decision, and best practice means immunising children," he said.
Last week, the Autistic Collective slammed Vaxxed as a sham that perpetuated false understandings of both autism and vaccines. Last month, the journal Altogether Autism said there was no causal relationship between the measles-mumps-rubella (MMR) vaccine and autism.
Paediatric Society chairman Dr David Newman told the journal: "The key fact is that even if thiomersal was linked to autism [which it is not], New Zealand childhood vaccines have not contained thiomersal since 2000, making it impossible that mercury in vaccines is in any way linked to the reported increase in autism diagnoses in New Zealand."
Child vaccines, which are free, begin when a child is 6 weeks old. The Northland DHB is hoping more parents will take up immunisation, noting that vaccination uses the body's natural defence mechanism, the immune response, to build resistance to specific infections.
Parents who have any questions are advised to talk to your doctor or practice nurse, or call the Immunisation Advisory Centre's free helpline on 0800 466-863.
Dr O'Sullivan was named Kiwibank New Zealander of the Year in 2014.
In 2013, he was acknowledged as an emerging leader by the Sir Peter Blake Trust, named a public health champion by the Public Health Association and named Maori of the Year.
In 2014 he was named by Reader's Digest as the second most trusted New Zealander, behind Victoria Cross winner Willie Apiata and ahead of former All Black captain Richie McCaw.
