G'day, Fredd. ET has nailed the best ones. Here's a few more:
Officer Cadet Museum Duntroon- he'd best email them on
duntroon.museum@defence.gov.au to see if they'll let him in, and when.
Army Museum Victoria Barracks Sydney-
aam.nsw@defence.gov.au and he'll need photo ID to get in.
Army Museum of Military Engineering at Holsworthy. He will definitely need to email ahead as it's a very restricted entry area and they will turn him away at the barracks gate.
aamme.mailbox@defence.gov.au Unfortunately 3RAR is no longer in Holsworthy- they had a very good unit museum, as did 8/12 Medium Regiment.
If he's willing to drive up to Singleton then the Infantry museum is excellent.
aaim.aahu@defence.gov.au and
www.infantrymuseum.com.au
Lastly, some of the RSL (equivalent to the British Legion) have small museums attached. If he asks the national HQ in Canberra they may be able to point him at some.
In a non-military/naval area there's the national dinosaur museum at Gold Creek, where he can see ET, Doug and Donald on display. It also has a nice walk-through aviary and cafes. Most of the larger towns have a community museum as well, but they can be anything from a couple of photos to the reasonable one in Batemans Bay. If he's a gamer the Batemans Bay club is on every second Wednesday night. The next meeting is 14 June, 1800 hours at the Custom Hire building (behind the Betta Electrical and gym building) on Cranbrook St.