You should be able to commence your maternity leave early, rather than resigning outright. If they require you to notify the dates of the start of your mat leave prior to that date, and your health is an issue, your Dr should be able to write you a sick certificate for the period between now and when that notice date occurs. You shouldn't have to resign just because your work is not appropriately flexible.

I strongly suggest you seek advice before you take any action. Your Union is a good place to start (if you're not a member you can join and seek advice on the same day) otherwise the Victorian Equal Opportunity and Human Rights Commission Ph 1300292153 should be able to fill you in on your legal rights and responsibilities in this situation.

ETA - yes you could resign and take sick leave until that point, but it may be better to take that leave and leave the situation to cool and give you some space to make that decision (ie so you're not under pressure), unless you intend to find another job before you have your baby.