You can log in to Tax professionals services with your Administrator AUSkey to apply online for your client's ABN. You will be required to complete a declaration at the end of the application.
Not everybody is entitled to an ABN. Although everybody can apply, only those who are entitled will get and hold an ABN.
When applying online for an ABN you will be asked to provide details about your client's business or organisation. Providing the right information may reduce delays in processing the application.
To apply for an ABN via Tax professionals services you need an AUSkey and to be the registered agent for your client.
When your client gets an ABN their details are stored in the ABR. There are options available if the application has been refused.