Most intending Australian immigrants don't use immigration agents. Australian immigration requirements are very straightforward and inflexible and DIAC is free of corruption (a concept that you may find hard to understand and accept). No agent can get you a visa that you would not have obtained by your own efforts and no agent can do anything to get you a visa any faster. Any agent who claims otherwise is lying and should be avoided at all costs.
If you insist on using an agent, make sure the one you use is MARA registered. MARA registration doesn't guarantee that an agent is competent, but at least they have access to all Australian immigration legislation and detailed requirements and have a code of conduct that must be adhered to in order to retain their registration.
https://www.mara.gov.au/default.aspx MARA website.
https://www.mara.gov.au/Consumer-Informa... Indicative agent fees
You should also join Pomsinoz which is the largest and busiest Australia immigration forum (open to everyone - not only poms) and search on there for recommended agents. You could also ask this question on there.
http://www.pomsinoz.com/forum/migration-... Pomsinoz forum