A pay equity lobby group is urging Parliament to scrap a bill that's part of National's support agreement with United Future.
The Taxation (Income Sharing for Tax Credit) Bill was drafted by United Future leader Peter Dunne.
It didn't get very far in the last Parliament and the cabinet isn't keen on it but Mr Dunne is insisting it goes further in the next one.
It would allow couples with children to add their income and split it down the middle for tax purposes.
Income sharing significantly reduces tax liability if one partner earns a lot more than the other because it puts income into lower tax brackets.
The Pay Equity Challenge Coalition says the only people who benefit from it are high earners - particularly men.
"The coalition is very concerned that with women's wage levels lower on average, it could result in more women staying at home or reducing their hours in the paid workforce while for most men it would be business as usual," says coalition spokeswoman Angela Mcleod.
"Income sharing returns us to the past with its stereotypes about women's roles and reinforces the gendered nature of child caring."
The coalition says it doesn't even begin to address real issues like child poverty and raising the income of low and middle income families.
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