Established in 1933, the New Guinea Club was a businessmen’s club with strict guidelines for membership, although that didn't prevent a young Errol Flynn from charming his way in. It was badly damaged in WWII and rebuilt in the 1940s to its former glory, only to be destroyed again by fire in 1993. It has been partly restored and is now home to a small and interesting museum.
There's often a fee collector or local historian in attendance, otherwise ask at the Rabaul Hotel for the key.