It's generally understood that sex is biological and gender is social, or grammatical: it makes sense for those two to go together as in many languages, including standard English, you need to know a person's gender in order to talk about them easily.
In the UK, at least, official documents specify "sex", not "gender". We may be slowly moving towards a world in which sex is officially recorded only in medical records, while gender is an optional field in social media accounts and the like.
All the forms in my country, especially the government ones, use 'gender' and not 'sex'. For me, when I see 'sex' on a form, it's jarring to say the least.
That's interesting. I wonder what's going on there. Do the words "sex" and "gender" have slightly different meanings for an Australian, or are the Australian forms asking a slightly different question from the UK ones?
But your passport says "Sex / Sexe" just below your date of birth, right?
The design of passports is rather precisely defined by an international standard. Interestingly, and perhaps relevantly, Australia is one of a dozen countries that routinely allows its citizens to have an 'X' instead of an 'F' or 'M' in that field. There is an ongoing legal case to obtain that right in the UK (Christie Elan-Cane).
Does your driving licence indicate "sex" or "gender"? A UK driving licence shows neither, and it doesn't give a "title", either: just full name, address, date of birth, and photo. In most cases that would allow one to guess the sex and gender of the holder, of course, but not always.
In the UK, at least, official documents specify "sex", not "gender". We may be slowly moving towards a world in which sex is officially recorded only in medical records, while gender is an optional field in social media accounts and the like.