You would appoint a lawyer to help you buy your house.
You would engage an accountant to help you with your tax return.
For the same reason, you should engage an experienced registered migration lawyer.
Professionally prepared visa applications are granted with minimal fuss, risk or delay. A poorly prepared application on the other hand can lead to delay, enquiry, and if it is rejected, a three year ban on re-applying.
The reality is that migration law is becoming increasingly complex, and you need an expert by your side.
Insist upon using a Migration Agent that is registered in Australia – that way you know that their conduct is governed by the Code of Conduct, and regulated by the Migration Agents Registration Authority (MARA).

See also: The Department of Immigration and Citizenship (DIAC) warned against using unregistered migration agents.

Discover more answers to your Australian Immigration Questions here!