Down around my area (Central Coast, just north of Sydney), there seems to be three groups: Cruisers, Scooter/Moped and everybody else. Everybody else will wave/nod back but it's rare to see a Cruiser or Scooter/Moped rider acknowledge. However as others have said when it comes time to stop for a break, they will mostly pop over and have a look and quick chat.
Also as others have said, blasting down the Old Pacific Highway you see groups of similar bikes riding together, probably because the ride was arranged through a bike club or forum similar to this one where it's mostly only people who ride that particular bike gather.
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