After 12 years of agitation by gay activists, Statistics NZ has published a paper acknowledging for the first time "emerging interest" in sexual orientation and behaviour.
A former editor of the gay newspaper Express, Victor van Wetering, has asked the Office of Human Rights Proceedings to take a legal case against the department on the grounds that not including a question about it in the census is illegal discrimination.
Census forms already let people describe another person in their household as their "same-sex partner" - a category that almost doubled from 3255 couples in the 1996 census to 6171 in 2006.
But census-takers here and in Australia, the US, Canada and Britain have all shied away from asking people whether they are heterosexual, homosexual or bisexual.
Statistics NZ's manager of social conditions, Conal Smith, said he was not aware of any country that had included the question in a census yet. But he said: "There is a legitimate user need for this information."
The discussion paper says health authorities need data on gay, lesbian and bisexual populations because they have higher rates of suicide, physical and verbal assault, bullying, depression, smoking, alcohol and drug dependence and workplace discrimination.
Other agencies are considering changes in welfare benefit entitlements for same-sex couples living together, adoption and family law, and hospital and school policies.
But Mr Smith said there were still two barriers to asking people about their sexual orientation in official surveys.
One was technical: sample surveys might not be reliable because gay and lesbian people were concentrated in a few areas such as central Auckland, which would have only a fraction of all the respondents in a national survey.
The second issue was public acceptability. Questions which even a smallish chunk of the population don't like produce very bad data.
"That can affect the quality of the rest of it."
Statistics NZ held focus groups before the last census that found that "a majority voiced acceptance or grudging acceptance to the idea of a question on sexual orientation being included in the census".
The greatest opposition came from Pacific people and to a lesser extent Maori, Asian, rural and older groups.
"There is concern that public resistance to a sexual orientation topic could result in reduced response rates for high interest groups," a paper says.
Mr Smith said Government Statistician Geoff Bascand had decided that there would be no new topics in the next census due in 2011, so any question about sexual orientation might be asked first in a sample survey.
