To get to Thirlmere by pubic transport:
This requires an early start. Take a Macarthur via Airport train to Campbelltown where you change to the Moss Vale train. This train connects with a bus at Picton. The departure time from Campbelltown is 8.50am. The latest connection departs Central at 7.50am. There are several departures from Central in the 40 minutes to 7.50am. The bus does not accept Opal cards but will issue a railway ticket to Thirlmere, A separate fare may need to be paid. Tell the driver you are visiting the Museum. The bus is only about 10 minutes to Thirlmere. The Museum opens at 10.00 so there is time to get a coffee from the nearby shops. Ask the Museum receptionist if there will be a visiting group that day. If so there will be a tour guide and you should be able to join one of the tours. Buses return to Picton at 1.20pm and 5.10pm. There are other buses to Tahmoor between these services. There is not a common bus stop in Thirlmere so you should ask for guidance.
This all sounds complicated but is not too bad. There are no bus services at weekends.
At Thirlmere there is a classic American style Buffet Diner car which is open for internal inspection.