How much does it cost to hire a digital marketing agency in Australia?

That is like asking how much it costs to buy a car -- it depends.  Do you want a Kia, Honda, Lexus, Jaguar or Rolls Royce?   They are all cars and can all get you from A to B but the experience will be different.  When hiring an agency what role do you want them to  play? Strategic only or execution too?    Have you conducted market research or are we starting from scratch?  Quantitative and qualitative?  Has your web site been optimized?  There is no one size fits all here it really just depends on your goals and what level of confidence  you need to make decisions and move forward.  Some may work on a project basis others will require a monthly retainer so find an agency that fits your needs.   

You don't always have to hire a digital marketing agency to achieve the results you needs. You can hire various people who special in each areas for a fraction of the cost. I always think it's a great strategy to create a team to support your business, so if one isn't available, all wheels won't fall off!  There are Facebook Ad, SEO, Social media and online marketing experts. And you can hire them per project. 

Depends on the types of services you're after.

Generaly my agency starts at $600/month for Google Ads manageent and then from $1500/month for really strong SEO.

As my clients have different needs, I chat with them about their needs and then put together a proposal for them. No one size fits all. All my terms and conditions are listed in the proposal document including how many revisions are included (mostly two for copywriting services) and payment terms (50% upfront). 
I offer per project or monthly retainers based on the work.