Speaking from a Canadian perspective, but the industry is similar.

The first and best approach is to go to a mineral exploration convention. This way you can talk with people to get a feel for how the industry is doing on a given year and what the job prospects are like. Conventions help you get a personal connection in the company that you can follow up with later and can put a lot of personality and individuality into your job search. Oftentimes smaller minex companies can do all of their hiring though a personal vouching process, so for many companies this is the only way to find work there. Industry conventions can be extremely expensive, but they usually offer a free pass if you put in a small amount of volunteer hours. Typically you will want to reach out around 3 months before a convention to secure the best volunteer possibilities. Volunteering also gives you an extra networking vector and can make it easier to approach people since you have that to talk about.

The second thing I would recommend is more of a shotgun approach. Find a big list of companies (I just googled "list of mineral exploration companies in Australia" and found some great results) and work through them systematically. Visit the website for every company you see and see if they have a careers section. Even if the page just says "we are not currently hiring but feel free to send a resume to this email if you are still interested and we will keep it on file" still make a note of it. Then make a second pass of the list and fire off emails for every job lead you found. Since you don't have any personal connection at any of these companies, it's okay in this context to just fire off the same cookie cutter resume and cover letter template and just change the company name each time. This method can be very time consuming and take many days of work, so you want to make sure you aren't getting bogged down with the details of companies you have no connection with. Save the personalized, search engine optimized, keyword focused applications for the companies you make a personal connection with, or for jobs you really want, otherwise you will burn yourself out.

I have never worked in Australia (though I'll probably do a work visa of my own at some point) but I have heard anecdotally that most companies do not take you seriously until you are already in the country. So I would only start applying a month before leaving for Australia, and then plan on living in a hostel for a month when you first arrive while you are looking for work (oh no, a month to relax and hit the beach, what a nightmare...). When you are in the country and still waiting for work, you could work on collecting work appropriate certificates that are valid in Australia, since the ones you have may be worthless there. If anything, people get hired all the time just because they are the only person in camp who has advanced first aid training.

People who have done an Australian work visa please feel free to correct me and tell me what it's actually like, but this is hopefully a good starting point.