The federal Nationals MP wants his colleagues to back a plan to reverse measures that deny fathers paid parental leave when their primary care partner doesn't satisfy a work test.
"Fathers should be able to bond with their babies," the regional party's special envoy for men and boys told reporters in Canberra on Thursday.
Mr O'Brien will use the Nationals' federal conference in Canberra on Saturday to push the case for change.
He has the backing of Nationals leader Matt Canavan, who sees it as an answer to Australia's declining birth rate.
"I'm very passionate about this because I want to see Australians have more children," Senator Canavan said.
Parents with a newborn are provided 26 weeks of paid leave at the minimum wage of $1004.90 per week.
Each parent has 20 days of leave and the rest can be shared flexibly.
However, access to the scheme requires each parent to have worked 330 hours in 10 of the 13 months before the child's birth.
Mr O'Brien said this cut out many single-income households with stay-at-home mums who were not employed.
"A father's parental leave should not depend on the hours a mum works in the formal economy," he said.
"Raising children is work too and families shouldn't be penalised when a mum chooses to make that her priority."
The Nationals' push for more leave is in stark contrast to comments from the alternative conservative party.
One Nation's Pauline Hanson was a single mother who ran a fish-and-chip shop before entering politics, and it has shaped her policy.
"If women take time off and they are not paid their wages because they're not working, fair enough," she told the National Press Club in July.
"Why should business pay?"
Senator Hanson has since clarified her support for maternity leave but remained adamant it could be costly for small businesses.
The birthrate was 1.48 babies per woman in 2024, according to the Australian Bureau of Statistics.
Government modelling shows Australia requires a birthrate of 2.1 babies per woman to sustain the country's future population.
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