I've no experience in that field, but in general seek.com.au is the main internet site for jobs in Australia. If you use that site, you can filter by category/sub-category (i.e. Science & Technology/Environmental and then search by job title ("Environmental" got 42 hits).
Scrolling through the advertisers for those postings may give you an idea of agencies that work in this field (I noticed one that has several posting).
186 visa - Temporary Residence Transition stream
Granted Dec 2013