Cheating husbands now face more than the cost of a divorce: their mistresses will also be able to get maintenance payments when things turn sour.
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Family Law Act reforms entitling de facto partners of two years or longer to the same rights as married couples stipulate that such a relationship can exist even if one of the partners is already married, the Courier-Mail reports.
Family lawyer Paul Hopgood said living together was not a requirement to qualify under the new rules.
"You don't have to live in the same house and under the same roof to be a de facto," the Courier-Mail quoted him as saying.
"A lot of people are living in de facto relationships and don't think they are."
The new Act, passed Monday in the senate, defines a de facto relationship as a couple regardless of sexual orientation "living together on a genuine domestic basis".
But Mr Hopgood said courts would accept explanations like travel commitments as reasons for recognising de factos not living under the same roof.
Queensland Law Society chair Julie Harrington said the laws would also hit polygamists who had only one recognised marriage but multiple de facto partners who now had some rights under the Act.
She added that men who had been in a succession of monogamous relationships could also face paying maintenance support to multiple ex-partners.
But a spokesman for Attorney-General Robert McClelland said people in this situation could show "just cause" to family courts for dropping any order.