Based on the information you have provided it would appear likely you are suffering from discrimination on the basis of your pregnancy for both the non payment of the bonus and your employer's apparent unilateral decision to deny you your old job at the end of your maternity leave.
The following links should assist your understanding as to your rights:
Only a fully briefed lawyer can give you firm advice as to the strength of your case, and your entitlement to compensation, if any, however, prima face you do appear to have a case and if you are keen to assert your rights you should engage an employment or discrimination lawyer to explore your legal options more fully. Strict time limits apply to such claims, so do not delay.
If you need help locating a suitable lawyer, contact the Law Institute of Victoria as they can refer you to a suitable lawyer in your area:
On the other hand, if you are happy in your new role, and the pay is sufficient such that the lost bonus is not really an issue, then you may be better off letting the matter slide, as litigation to enforce your rights will likely sour your relationship with your employer.
I trust the above assists.
Good luck and please rate my answer.