Department of Social Services officials told a Senate estimates hearing on Friday that it needs to confirm exactly which forms of volunteering will be considered as part of the new work, training or study test for the Abbott government's childcare reforms.

The Coalition's childcare package announced in the May budget will require parents that work or do a work-like activity for at least eight hours a fortnight to qualify for up to 36 hours of childcare subsidy over the same period. They will have to work at least 49 hours to get the full 100 hours per fortnight subsidy.

In the current system, many parents are able to access payments without working at all.

While volunteering is currently part of the work-test for some existing childcare payments, early childhood deputy secretary Jackie Wilson told Senate estimates the department needed to "have a good discussion about what is actually a stepping stone for work and workforce readiness".

Secretary Finn Pratt said that while "lots and lots" of volunteering activities would be acceptable, there could be "some things in the grey areas".

The childcare sector has expressed concerns that thousands of Australian families may have to pull their children out of childcare because of the new workforce participation requirements.

Bodies such as Early Childhood Australia and major childcare providers are concerned that parents, including casual and seasonal workers and new mums trying to break back into the job market, will not be able to afford to keep their children in childcare.

While Social Services officials were asked how many families would lose out on childcare as a result of the changes - due to begin in July 2017 - they told senators they would have to "take it on notice".

Ms Wilson also told the committee that the department was looking at introducing smart technology - such as an app - to help parents keep a track of their work or approved activity hours and therefore how much childcare subsidy they would qualify for in the new system.

Mr Pratt added that officials were trying to make the new system the "least onerous" it could be. He suggested that parents' work hours would not necessarily be judged week to week but as an average over a longer period, say three months.

Social Services Minister Scott Morrison has stressed the new childcare package - which provides an additional $3.5 billion - is intended to "help parents who want to work or work more".

The government has also announced a nannies trial, beginning in 2016, as part of its package.

Mr Pratt said money had been set aside to keep the nannies program going, if the trial was successful.