The Turnbull government expects 230,000 people will increase their hours or go back to work when its revamped childcare subsidies come in next month.
But three weeks before the changes begin nearly a third of families eligible for the increased payments haven't made the transition to the new system.
From July 2 families could be up to $1333 a year better off but only if they update their details through myGov.
Federal Education Minister Simon Birmingham says 800,000 families have made the switch, but more than 1.1 million families could be eligible.
He says the increased subsidies are about helping women hampered by the cost of childcare to work.
Around 230,000 people will increase their workforce participation, government modelling shows.
"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," he told ABC radio.
The changes include rebates targeted towards families working, looking for work, studying or volunteering.
Means-tested rebates would also see low-income earners receiving up to an 85 per cent subsidy for their childcare costs compared with 72 per cent previously.
HOW AVERAGE AUSTRALIAN FAMILIES WILL BENEFIT:
* A family on $50,000 - both parent/s working, with two children aged under 6 in long day care two days a week at $100 a day will be around $2,000 better off a year.
* A family on $80,000 - both parent/s working, with two children aged under 6 in long day care three days a week at $100 a day will be over $3,000 better off a year.
* A family on $150,000 - both parent/s working, with two children aged 6 and under in long day care three days a week at $100 a day will be more than $1000 better off a year.