If you don't get any joy from EOWA try HREOC - Human Rights and Equal Opportunity Commission. They should be able to give you some advice. It is illegal to pass someone over for promotion due to parental leave so I would assume that the pay increase would be similar, especially seeing that you only took 3 months (which would be equivalent to some people's Long Service Leave). Definitely look into it further because it doesn't sound right to me.