There are no legal responsibilities for telling prospective employers that you are pregnant, unless there's a possible OH&S concern - like if the job you're applying for involves being exposed to chemicals, or something that might prevent you from doing the job because you're pregnant.

An employer cannot use your pregnancy as a reason not to give you a job, and they should definitely not ask you whether you're pregnant or planning to have a family, during a job interview - that's discrimination. They can ask you if there is anything that might prevent you from doing the job, and pregnancy might be a factor in that (for example, working with cats in a vet clinic might be something you want to avoid whilst pregnant - you should tell the employer about that).

If you're already in a job and some of the tasks are things you can't do/want to avoid because of the pregnancy (cleaning out the the cat litter, for example), the employer needs to make reasonable changes to accommodate that, just like they might accommodate someone with a disability. But what are 'reasonable' changes is debatable.

Technically you don't even need to tell any employer that you're pregnant even if you're obviously showing a baby bump - the only time you have to tell them is when you put in an application for maternity leave.

GL!