AN anti-vaccination group is encouraging parents to circumvent the NSW government's crackdown on unvaccinated children by joining a "dubious" religious organisation.

The Australian Anti-Vaccination Network (AVN) is telling supporters to join the Church of Conscious Living to get their children into preschool.

"The tenets of this church absolutely oppose forced medication including vaccination," the AVN says on its website.

It's promoting the church as an option for parents who don't want "to join the Church of Christian Science in order to get their children into preschool or childcare".

NSW Opposition Leader John Robertson has questioned the credentials of the church.

"The credentials of the Church of Conscious Living as a genuine religious organisation are completely dubious - yet its members will be able to use it to gain an exemption," he said.

Unvaccinated children will be banned from childcare and childcare centre operators will face fines of $4,000 if inspectors discover they are caring for children who don't have proof of vaccination, under new state laws.

But the legislation, introduced to parliament on Wednesday morning, leaves the door open for parents objecting to vaccinations on religious or medical grounds.

Mr Robertson said the government had created a loophole large enough for anti-vaccination supporters "to drive a truck through".

"This loophole created by the O'Farrell government is a victory for the AVN," he said.

Mr Robertson called on Health Minister Jillian Skinner to explain what the government would do to close the loophole.

"It's clear that members of the AVN and their supporters will use any means available to avoid protecting children from whooping cough and measles," he said.

Asked about the matter in parliament on Wednesday, Ms Skinner said the Health Care Complaints Commission had launched an investigation into the AVN.

"(It) has been monitoring the Australian Vaccination Network ... where they believe that a body such as the AVN is acting counter to the health of people they can take action," she said.

Under the government's legislation, parents who want to exempt their children from vaccination will need to provide a certificate from their GP or an immunisation nurse, after undergoing compulsory counselling.

The government's crackdown comes just over a month after a report said tens of thousands of children were behind in their vaccinations, with fears a potentially deadly outbreak could also affect immunised children.

Originally published as Anti-vax parents may join 'dubious' church