Do I need to apply for an assessment?
There is no legal requirement that separated parents must apply for child support.
You can work out child support arrangements between yourselves. However, if communication breaks down and things go wrong with the other parent, there may be nothing you can do to enforce payments under an informal agreement with the other parent.
You also need to apply for a child support assessment or have a legally recognized child support agreement in place to get more than the minimum rate of Family Tax Benefit Part A. There are exceptions to this rule such as where you may be experiencing family violence or you are unable to locate the other parent.
If you are receiving no financial support from the other parent, absent any agreement then the only way to force payment is to seek a child support assessment. Importantly, if you have never applied for a child support assessment previously, there is no way to get back paid child support.