You DO NOT need to use immigration agent to apply for visa to migrate to Australia.
They have no influence whatsoever and does not increase the chance of you being accepted. Basically they just act as repositories for your document before before sending them to Canberra.

Go to immi.gov.au website. Collect the forms pertaining to migration, fill it all and send it yourself.
If you really need to use a migration agent, if possible use one that has a local office here. Make sure they are registered by MARA (https://www.mara.gov.au/)

Cheers
SK