To register a .org.au domain you must be a not for profit organisation.
You can use the ABN of the umbrella organisation for the charity as long as the charity is registered in that organisations name.
For Registrant details, you can use the umbrella organisations name and ABN. For an Eligibility Name, enter the eligibility charity name and for Eligibility Type select Charity.
This order shall be verified manually, but it is usually processed very quickly.
Note: If you can't find an answer to your problem click Here to open a support ticket (requires log in).