There are several different kind of markers:
- National Engineering Landmark
- Historic Engineering Marker
Use the tag "engineers australia" to search for them or use this link: geocaching.com.au/tags/view/all/engineers australia/
At https://docs.google.com/spreadsheets/d/1nBYGitYPdSK3cOJdBsHCINMEN4RFX24QBfgOGn6xoyY/ is a copy of the spreadsheet used to keep track of which locations have been determined.