The Australian Public Service Commission discussed the main findings of its just completed research at a Senate estimates hearing on Tuesday which included that women are now being promoted in the public service at a faster rate than men, except for the year after maternity leave.

Australia's stubborn gender pay gap sits at 14.1 per cent.

It comes after the Albanese government introduced legislation last week to force medium and big companies with 100 or more workers, as well as similar size public sector agencies, to reveal their gender pay gap.

Assistant Commissioner Dr Nicole Steele said the APSC looked at differences in salary when staff are at the same classification level.

"What we wanted to ascertain was that if we paid everybody the same base salary for that classification, would that drop the gender pay gap?" she told the hearing.

"What we found is, it didn't. It dropped the gender pay gap from 6 per cent to 5.7 per cent."

But she said, under questioning from Greens senator Larissa Waters, when the proportion of women at the lower classification levels was looked at, it was found that it was significantly driving the gender pay gap.

"So what we're finding is, we have a very high proportion of women at the lower classification levels. If we balanced that out with men, the gender pay gap would reduce from 6 per cent to 0.4 per cent. So that's quite significant," she said.

"What we've actually found in the last 10 years, women are being promoted at a faster rate than men except in the year after they take maternity leave. So the maternity leave year, other than that, women are being promoted at a faster rate."

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But promotions are not the whole answer.

"It's actually about the men. Getting more men at the lower, or balancing out men and women at the lower levels," Dr Steele said.

"If we actually keep promoting, we actually see the gender pay gap actually increasing. It's more about the disproportionate number of females at the lower end of the scale that's driving it."

Which then gets to why more women are at those lower levels.

"It's a very complex issue. It could be a societal issue as well. And what's driving women to be at that level," she said.

Minister for Women, Katy Gallagher, is yet to have a full briefing on the findings, but issued caution about the task ahead.

"I want the APS to be a leader. We already are, in terms of the gender pay gap, is much smaller," the senator said.

"The big issues are outside of the APS, really in terms of closing the national gender pay gap, which is at about 22.2 per cent.

"But I wouldn't underestimate the challenge of re-balancing at that lower end. I think when you look at the big employment areas, I imagine it is Centrelink."

Dr Steele added, "It is. Service delivery areas. In the regions."

The APSC findings have been sent to the Workplace Gender Equality Agency and the Office for Women.