Australia's near neighbour became the first Asia-Pacific country to legalise same-sex marriage on Wednesday after a decades-long campaign, with parliament voting 77-44 in favour.
It made New Zealand the 13th nation to allow gay marriage, according to Human Rights Watch, with Britain and France both expected to adopt similar legislation in the near future.
In contrast, Australia's parliament last September voted overwhelmingly to reject the move, and the unmarried Gillard said the New Zealand breakthrough would not change her mind.
"On same-sex marriage, marriage equality, I doubt we're going to end up agreeing, sorry," she told a community forum late Wednesday when asked why Australia did not follow the New Zealand example.
"But what we have done as a political party is we've decided that people can exercise their own views, own conscience, when this matter comes before the parliament, which it recently did and I'm sure it will again in the future."
In the September vote, the Australian House of Representatives rejected a bill to legalise same-sex marriage by 98 votes to 42.
Both Gillard and staunchly Catholic opposition leader Tony Abbott, widely tipped to win national elections later this year, voted against it.
Gillard allowed Labor MPs a conscience vote on the issue -- meaning they were free to vote how they wanted rather than along party lines -- while the opposition did not.
"The matter has been dealt with by the current parliament. It was fairly decisively rejected," Abbott, who has a lesbian sister, told reporters Thursday.
"My own position on that particular question is pretty well known."
Advocates of equality say gay marriage has broad support in Australia, where same-sex unions are recognised in five states.
However, because marriage is covered by federal legislation, which defines it as only between a man and a woman, couples joined in civil unions are not seen by the national government as married.
Despite Gillard and Abbott's stance, Australian Marriage Equality national convener Rodney Croome labelled the New Zealand vote a "game changer" because of the close links between the two countries.
"I believe that any politician who ignores support for this in the community will lose votes as a result," he told ABC radio.
With gay marriage not recognised in Australia, Croome said he expected as many as 1,000 same-sex couples to cross the Tasman Sea to get married.
"Now that marriage equality is only three hours away there will be a flood of couples flying to New Zealand," he said.