The government says it is not good enough that parents expect taxpayer funding yet don't vaccinate their kids.

The federal government has flagged a possible welfare crackdown on parents who fail to vaccinate their children.

It comes as government figures show the number of parents choosing not to vaccinate their children as "conscientious objectors" has doubled since 2006.

Parents must immunise their kids to receive childcare benefits or the Family Tax Benefit Part A, however exemptions exist for medical reasons, conscientious objection, or through a letter explaining why the exemption is necessary.

Social Services Minister Scott Morrison says there are loopholes that need to be closed, signalling measures in his much-anticipated families package.

"We're a free country and we're a free society but that doesn't mean you get to take taxpayer's money if you don't want to go along with the rules," he told Macquarie Radio.