Basiclaly, you need to be the sort of person who QF deems to be extremely worthwhile to keep on side - MP's, Senators, heads of government departments, CEO's of major corporations, etc., etc. Platinum FF's do not get into the Chairman's Lounges.
They only seem to be at domestic airports - MEL is at the top of the escalators into the QP, where you turn left at the top to go to reception, you will see some frosted doors immediately in front of you. SYD T3 is located in the walkway between East and West Receptions, CBR is near gate 14 (before you enter the QP, BNE is just near the "finger" gates. Not sure where the others are.
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The Sydney Chairman's Lounge can be reached by walking straight ahead when you get to the top of the escalator, instead of doing a hard right and going into the Qantas Club, the Adelaide CL is on your right (with a door marked 'Private'). (incidentally the Adelaide CL is considered the worst in Australia and, unless it has been updated since the last time i was there, looks like something straight out of the 70's in terms of decor, etc)
Darwin, Hobart, Launceston etc don't have Chairman's Lounges, members have to use the normal Qantas Club facilities.
As for what goes on behind the frosted doors and how it compares to the normal QC...not much different really. You have your own top shelf, self service bar, better food and the staff come and fetch to board your flight, but apart from that it is pretty similar to the Qantas Club.
I haven't used an international OW F lounge before but I imagine that it is pretty similar fit out, facilities and food and drink as a Chairman's Lounge.
Actually, there is a level above CL - see this thread at Flyertalk.
BNE CL is between Gates 22 and 23, just a door marked "Private".
Simon, the Intl F lounges are better decor-wise as well as sizing, than Dom CL's (Yes, I've been in CL's before, my godfather was CEO of an industry body). I have had the pleasure to be in the F lounges in BNE, SYD, MEL, AKL, BKK, SIN, HKG and LHR as well s the new FL lounge in LAX.