It is really tricky to tell, because it will all depend on your terms and conditions of employment &/or workplace agreement.....

Do you have the paperwork you originally signed when you first got your FT job? Does it have any detail/small print? Were you provided with any ammendments to this at any point, particularly before leaving for maternity leave? Do you recall your employer providing you with a maternity leave statement/paperwork?

Check out this link HERE that details the current workplace legislatation.

Under normal circumstances it sounds like what your boss has said to you is illegal, but it is tricky to know without knowing your specific workplace agreement. (I hate to say it, but they may be either ignorant of the legislation, or trying to pull a fast one: both intolerable.)

The company that you work for.....does it have an Human Resources Dept that you could call?

Either way, I hope it sorts itself out. The situation they have put you in is outrageous. Best of luck.