"When I saw my half-brother's homophobic ad, my thoughts went straight to the young Australians who will see this on television, telling them that they don't belong, that they're somehow inferior," he says.
"Our nation's leaders should not be telling the community that it's okay to discriminate. It's not okay for politicians to get away with hatred and bigotry."
The Australia Party - founded by Bob Katter, the federal independent MP - is contesting its first election in Queensland. With its conservative social values and its pitch to disenchanted rural voters, it has been likened by some to Pauline Hanson's One Nation party.
In its TV commercial, which has been criticised by the Queensland Premier Anna Bligh as "way, way beyond what we expect in a political campaign", the party warns voters that Newman cannot be trusted on the gay marriage issue.
While Newman personally supports gay marriage, he has said he accepts the official LNP line, which opposes new Queensland legislation allowing civil unions for gay couples.
The Australia Party ad has been condemned by four of its own candidates, as well as by the federal shadow treasurer Joe Hockey, and the former Liberal Party leader Malcolm Turnbull.
Carl Katter, a board member of the lobby group Australian Marriage Equality, said the ad was "an insult to Queenslanders, [who] are so over being portrayed as bigots and homophobes".
He added: "Are we living in the 50s now? These people wear religion on their sleeve; I can't think of anything more un-Christian than this kind of victimisation they've undertaken."
The half-brothers have not spoken for many years. Bob said recently that he admired Carl's "candour, his forthrightness". But the latter told Sky yesterday: "I don't think he really does respect my involvement in wanting to create a fairer and equal Australia."
GetUp! wants to air its ad as much as it can before the March 24 election.