Aussienanna "Gender" is all the trimmings that surround the role of the sexes, - the way they feel about being male or female, how they usually dress and act, what is expected of them socially. That varies from one person to another and can indeed be changed so that the person presents (to themselves as well as others) as the opposite sex.
However, their sex is in every cell of their body, and in whether they contribute eggs or sperm to the making of a new human being. That cannot be changed, whatever is done to the body externally or what happens to the internal feelings. The number of people born intersex is a very small proportion of the population.
Socially anyone can live how they wish. The problems begin with a catch-22. Anyone can, without altering their body and its reactions at all, decide by self-ID that that they are now not men, but women, and can access any space or facility reserved for women.
However, anyone who questions whether an individual who claims this really has applied for a certificate of gender re-assignment runs the risk of being accused and charged with harassing that individual by invading their privacy.
There is thus no way of knowing for certain that they do have that certificate until it has been seen, and there is no obligation on them to actually produce it, and there is a penalty if the challenge is made to someone who then does prove to have a valid certificate. This means people are reluctant to raise a challenge.
Any Tom, Dick or Harry could walk into the communal changing room of a ladies dress shop, pretend to be trying on clothes, and enjoy the free striptease show taking place around them. An 18-year-old got a lot of publicity recently by doing just that - and by getting a public apology from the shop after an assistant made him wait for space in the crowded changing room instead of giving him preference. He was later said to be definitely male (and happy to stay that way) and doing what young men (and old) have been doing for thousands of years - getting into forbidden territory to watch women take off their clothes.
It is mostly this kind of ambiguity that makes some people object to legislation without intensive and honest debate - which is being stifled by the current exhortations to "be kind". Being kind is good - but legislation must cover and restrain those who might take advantage of that kindness.