How many households will be worse off overall after a shake-up of family tax breaks and child care benefits?
It's the question Labor and Greens senators were hell-bent to have answered when social services bosses fronted a hearing in Canberra on Thursday.
Despite their persistent line of questioning, though, a direct answer wasn't forthcoming.
The Turnbull government wants to pump an extra $1.6 billion into childcare fee subsidies.
It has tied the cash splash to a raft of welfare cuts including phasing out annual Family Tax Benefit supplements while slightly boosting fortnightly payments.
Social Services boss Finn Pratt cautioned senators against trying to calculate how many could stand to lose under changes to tax breaks without factoring what they would gain through child care reforms.
But, while there is a significant overlap between the two issues, his department only provides advice to government on its areas of responsibility, which includes the Family Tax Benefits.
The changes to child care are being steered elsewhere by education and training officials.
"We only look at what we are responsible for and of course the government's package - as we've said a few times - includes a range of other measures," Mr Pratt told senators.
Regardless, Labor senator Murray Watt was determined to dig for a "global" picture of the fallout.
"Has anybody provided ministers with advice about how many people overall will be worse off from these changes?" he asked the social services heads.
Liberal Senator Scott Ryan soon jumped in, arguing the bureaucrats fronting the hearing could only answer questions regarding their own department.
"So ministers are going ahead with these changes despite knowing how many families will be worse off?" Senator Watt said.
Senator Ryan quickly stomped on the grenade but Labor Senator Louise Pratt pressed on.
"It seems pretty extraordinary to me that you wouldn't model the child care impacts relative to the impacts on people's family income," she said.
"That's what we're expected to do in parliament, as they're all rolled into the one bill."