Unfortunately your problem is a common one, whereby the parent with primary custody restricts the other parents access as a way of extorting extra payments from the other parent. CSA cannot help you with this as their involvement extends only to ensuring child support is paid. The only way you can deal with this is apply to the court. If you already have parenting orders or a formal mediated agreement in place then you need to apply to the court for enforcement orders, and if there are no such orders in place, you will need to attempt mediation before a court will deal with an application to the court. To initiate the mediation process, you should contact an organisation such as Relationships Australia who conduct mediations of the sort required for a relatively modest cost. You can read about this here:
Once you have attempted mediation, or if your ex refuses to participate, you will be able to apply to the family court to decide your custody dispute.
Whilst the expense and headache of making such an application is not something to look forward to, once orders are in place your entitlement to access your children at definite times will have legal status and if she breaches the orders by withholding access or custody she will likely face legal cost orders if you again have to bring the matter before the court.
Also be aware that if you can show that the only reason she denied you 50/50 access was that she was worried about losing some of her child support, the court may be open to granting such increased access.
Also note that whilst legal representation is recommended, it is generally not necessary for the mediation process, and it is not impossible to bring your own court application without a lawyer, and whilst far from optimal, many parents are in your situation and the court is used to dealing with people who are unrepresented and generally strains to ensure they have a fair hearing despite their lack of representation or legal knowledge.
I trust the above assists.
Good luck and PLEASE RATE MY ANSWER.