If I am to venture a guess, it's a mentally change.
It's my impression people on the east coast are more big-city like in mentality, while in the south-west people are not that easily excited and certainly don't show it as much! And also have more reserbation about what they see as intruding on M&F. So people will notice M&F but leave them alone. - As a Jutlander I think I would feel very much at home on the south-west coast. It's a similar mentality I think.
Am I completely off the mark?
I don't think that sort of generalisation is safe. There is no one east coast mentality. If anything there is city v. country, but even that's simplistic. The east coast has lots of different sorts of settlement, and different sorts of mentality to match. There are big cities to smaller cities to coastal rural towns to tourist resorts of different sorts to fishing villages, and more.
I don't know where they were travelling in WA. Perth is a fairly large city, but it's a long, long away away from the east coast and, though I may be wrong, I think the difference is more likely to be that the people of Perth have no idea who Mary is, or at least not the same level of interest as people in the east. Mary is a Tassie girl and lived in Sydney for some time, so there's more interest in her on this side of the island.
Byron Bay is not a city, it used to have a whaling station and was a centre of the local dairy industry. It is a very beautiful part of the world though with rainforest hinterland associated with the extinct volcano, Mt Warning, and beautiful beaches and rivers and today it's a vibrant holiday destination all year round. There are always lots of European tourists, particularly Germans, and a large number of backpackers. There are also a lot of people in the area who have moved up from Sydney and Melbourne, mixed in with the long-term locals and the hippies who moved into the region in the 60s. There are lots of artists and artisans in Byron Bay, and it has some great restaurants. Byron Bay makes much of its natural wonders and marine park, and is very "green".
The Gold Coast is very different. It is also a tourist destination, focusing on the beaches, but seems to attracts a different sort of tourist. The Gold Coast is riddled with high-rise apartment blocks near the beach. That style of development at the Gold Coast would never be approved at Byron Bay. There is also the casino and shopping malls. It has a more city-like quality and a huge permanent population, including lots of retirees from southern states in that area.
Mullumbimby, where they were today, is different again. I was up there on Saturday. It's not far from Byron Bay but it has a different vibe. It is more a village servicing a rural community than a tourist destination. It is one of the places tourists staying at Byron visit on day trips. It's a charming town and does not have a city feel at all.
There will always be exceptions, but I think that most Australians who did recognise her - and I say "her", because I think it is Mary who is of interest here because we still see her as one of us, who just happens to have married a Danish Prince - would notice her and her family but leave them alone.