im not exactly sure that you are strictly "legally" entitled to expressing breaks. It would really depend on your employment contract terms and what your work arrangements are - full time, part time etc - contract or award etc and what state you are living in. I know the NSW public service has just negoiated a deal for women to have expressing breaks - however if an employer did not allow you to express - you would possibly have a discrimination case on the basis of being discriminated against due to you not being allowed to express.

i would try and negoiate with your employer and inform him prior to your return of your intention to express - if you have any problems and i was you i would contact the anti-discrimination commission in your state and have a chat to them. i would be armed with what your rights are prior to having that chat.

hope this helps,

best of luck -