There are a few reasons that you might need to use a bathroom that is not 'your' bathroom. For instance, maybe you are a parent and you have a child in a pram that does not fit into the stall and you want to use the disabled bathroom. Maybe you are a Dad and the only change table is in the women's bathroom. Maybe you have a child with you who is not your gender, who is old enough to use the bathroom but you don't feel comfortable sending them in there on their own. Maybe there is something wrong with one of the bathrooms, it has been vandalised or the toilets are not working. What is the legal situation here?
What is the law?
There is actually no law covering who must use which bathroom, whether on the basis of gender or disability. So the short answer is, use whichever bathroom you like.
If someone does something inappropriate in a bathroom, such as video record somebody, take photos, make lewd comments or gestures or leer at people then these are caught under other laws, like offensive conduct. So if you are in a bathroom and you are being 'offensive' then there is a law prohibiting you from doing that.
Some practical problems
Say you are a woman and you are out and about with a seven year old boy who wants to go to the bathroom and you are in an unfamiliar place, or worse still a four year old who looks like he is seven years old. You don't feel safe letting him go to the men's on his own, and he is refusing to walk into the women's room with you. Can you walk into the men's bathroom with him? The immediate pragmatic problem is men's bathrooms have urinals. This might be both the reason you don't want the boy in there on his own, and the reason that your behaviour may be offensive.
The definition of offensive conduct is intentionally vague, which is both good and bad. So think about how you might do what you need to do in the least offensive fashion. One thing you might do is stand at the entrance of the bathroom and say something like, "I am sorry, I have to come in with my son, is everybody decent?" If you didn't do this you probably wouldn't be behaving in an offensive fashion, but doing it might also head off an argument or any shock.
This would be less of a problem if you are a Dad in a shopping centre and the change table is in the women's room because women's rooms don't have urinals. However you are probably more likely to be criticised for doing this. Without wanting to get into a discussion about why that might be the case, just bear in mind that you should probably equally take precautions not to be 'offensive' when entering the women's bathroom even though there are no urinals.
You might say, parents should use the disabled bathroom. Well most people who are disabled, or live or work with people who are disabled, would tell you that they would prefer that you walked into the opposite gender's bathroom. This is basically an area where there are no clear rules or laws and you need to exercise your better judgement.
There may be other issues, for instance the shopping centre you are in might have rules allowing them to kick you out of the Centre for using the incorrect bathroom. Or, a person may start an argument with you and this isn't something you want to expose a child to. Whilst the strict legal answer is you can use the bathroom, in practice you may still elect not to do this.
What can I do about the problem?
The most effective person to lobby about this problem is probably your local Council, because local Councils decide on Development Applications. If it was (for example) a requirement for DA's in your local area that all public amenities include a disabled bathroom and a separate parents room then this would deal with most of the problems listed above. Will your local government do that when developers want everything done as cheaply as possible? Possibly not, but they definitely won't do it if they don't think people in the local community care about it.
If you would like to read some more of our articles on the topic of local government, then click on the button to the right.