Around 350,000 families who are eligible to receive the new childcare payment may not get a cent — because they are yet to fill in the necessary online forms.

Despite the package being introduced to Parliament in 2016, one in three families still has not signed up to the new system, which starts on July 2.

Federal Education Minister Simon Birmingham says more than 800,000 families have switched to the new subsidy, but many others are waiting until closer to the start date.

"Those who have switched so far are, on average, around $1,300 per child, per annum, better off under these reforms. We're urging people to get the job done," the Minister told AM.

So if up to $1,300 is on offer, why aren't people signing up?

Busy families trying to juggle work with child care may not have gotten to the paperwork yet, but they would be receiving reminders.

"We have written, emailed, text messaged and of course run a mass media communications campaign ... we do urge the remaining families to make sure you register so there's no disruption to your payments," Senator Birmingham said.

A three-month "grace period" will be available for those who don't complete the forms by the start of July.

Do you get paid? It depends on 'behaviour change'

The new Child Care Subsidy replaces two existing payments — the Child Care Benefit and Child Care Rebate.

The new payment will be subject to a means test and an activity test, which means parents will have to be working, volunteering, studying or looking for work in order to be eligible to receive the payment.

Critics point out that 230,000 families will be worse off under the scheme, including many who are vulnerable.

But the Minister says those models do not "assume any behaviour change."

"If more people choose to increase their participation in the workforce, then there will be even more families who benefit," he said.

"We make no apologies for structuring these reforms to give the greatest support to families working the longest hours ... and ... the greatest levels of financial subsidy to families earning the least amount.

"Our modelling shows that around 230,000 people will increase their participation in the workforce.

"For some, that will be choosing to work more hours or more days. For others, that will be choosing to go back to work for the first time ever."

There are still three weeks before the first payments go out, and Senator Birminghan says he is confident people will get on board.

"I've done it personally. It's a very straightforward process. It only took about 10 minutes sitting on the couch to do so."