Scots-born May-Welby is celebrating after finally being granted non-specific gender status by the Australian High Court in Sydney, 25 years after undergoing a sex change to become female.
The 52-year-old is originally from Paisley, Renfrewshire, and has lived in Australia since the age of seven.
He underwent gender reassignment in 1989, aged 28. Unhappy as a woman, however, May-Welby stopped taking hormones after the surgery and no longer identifies as male or female, choosing instead the term "spansexual".
May-Welby was given gender neutral status in 2010, but this was withdrawn months later by the New South Wales state government, starting a series of appeals that eventually reached the High Court.
The judgment handed down in May-Welby's favour yesterday was unanimous and stated categorically that "not all human beings can be classified by sex as either male or female". There is nothing in New South Wales legislation that requires the registrar to record someone's sex inaccurately as male or female.
After the court ruling, a jubilant May-Welby said: "Maybe people will now understand there are more options than the binary, and even if a person is specifically male or female, their friends might not be, and hopefully people might be a little bit more accepting of that."
May-Welby's lawyer, Scott McDonald, said the ruling sent out the message that the High Court does not believe gender is limited to male and female.
He described the case outcome as a "persuasive authority" for other jurisdictions and Australian states.
He said: "This is a very positive outcome for Norrie and others who don't identify as male or female. We now know that our existing laws are capable of including and recognising sex and gender diverse people."
Despite its ruling, however, the court repeatedly referred to Norrie as "she" and "her" in its judgment, noting that May-Welby's legal representatives used the terms in submissions.
The court was asked to consider whether "non-specific" can be included as a third gender category under Australia's Registry's Act.
Lawyers for the Registry argued confusion would occur from the acceptance of more than two gender categories, and that the purpose of a reassignment procedure was to assist a person to be considered a member of the opposite sex.
Nathan Gale, of the Scottish Transgender Alliance project, which has been funded by the Scottish Government Equality Unit since 2007, praised the move.
He said: "We greatly welcome the decision to accept someone's gender identity even where that identity is neither male or female.
"It's part of a growing movement around the world that recognises people have a variety of gender identities, and we hope that more and more countries will follow.
"There are two places in Scots law where this is being recognised: the Offences (Aggravation by Prejudice) (Scotland) Act 2009, where prejudice relating to transgender identity is an offence; and the more recent amendment to the Marriage and Civil Partnership (Scotland) Bill, which if passed will remove the last major piece of sexual orientation discrimination from Scots law and have the potential to deliver equal partnership rights for trans and intersex people."
The Bill currently requires all ceremonies conducted for legally mixed-sex couples to include a declaration that they "accept each other as husband and wife."
The Transgender Alliance proposes a further amendment that would enable mixed sex couples to use the gender neutral form of wording "we accept each other in marriage."
Mr Gale said: "It should be possible to allow a choice of gender-neutral or gender-specific language for marriage declarations. We'd also like to see Scots law enable people to have passports and birth certificates with no gender markers."
Patrick Harvie, the Green MSP, said: "I welcome the Australian High Court ruling and the recognition that gender is something people can define for themselves. Gender should not be something that society imposes in people. Identity is individual and distinct, and gender encompasses so many nuances. It is too easy to describe someone's sex in simplistic stereotypes, a limiting binary idea of human identity.
"This new ruling allows people the freedom to think for themselves. Other cultures are much more relaxed about a third sex or gender neutrality, but in modern Western society it's something we're just beginning to have discussions about.
"Scotland and the UK could learn a lot from having a legal system which recognises that identity can go beyond the binary notion of man or woman, and I would hope that local authorities too would also allow for people who don't identify themselves as either gender."