No way to casual and they can't 'make' you go casual. If they tried to do that at 27 weeks it would clear they are trying to avoid their obligations in relation to maternity leave. Threaten to go to the media and definitely contact a union, the AMWU (Australian Miscellaneous Workers Union) could be a good place to start. Don't worry about not being a member, you will still receive advice and they would love to take on an issue like this in the current political environment.