As a postscript to my article on Dual Nationals, I subsequently came across a couple of extracts from Australian Government websites.

First the Australian Government's Citizenship in Australia Fact Sheet 187 which explains 

Throughout the 1960s, Australian citizens were still required to declare their nationality as British. The term ‘Australian nationality’ had no official recognition or meaning until the Act was amended in 1969..... It was not until 1984 that Australian citizens ceased to be British subjects.

So it seems that Australian riders like Charlie Monk and Peter Vandenberg and others were technically British in the 60s! And if British, then what better country for Monk to choose than Scotland!

While the Department of Immigration Notes on Dual Citizenship state that

Prior to 4 April 2002, Australian citizens who became citizens of another country lost their Australian citizenship automatically.

Even after 1969, it is possible that, by spending the bulk of the year in the UK, Aussies may have taken out, or have been required to take out, British papers, causing them to lose their rights to Australian nationality. It was believed that Aussies Charlie Monk and Chris Bass both rode in Australia v GB tests at Adelaide in the early 70s while holding British passports. 

Curiously Edinburgh's Bill Landels, who emigrated to Australia at the end of the 1967, was selected to ride for the Aussies in the first test at Perth in November 1970, having made his debut for his new country at his home track in Sydney in January of that year. However questions were raised over his eligibility at the practice on the day before the meeting, and probably by locals rather than the tourists. A phone call to Owen Bateman, one of the Australian selectors and the promoter at Sydney brought the advice to use Les Sharpe instead. A strange compromise of naming him as GB reserve just fanned the flames of the controversy and ultimately a disgruntled Bill returned to his Sydney home without appearing for either side. He continued to turn out for Australia, but only at Sydney under Bateman's supervision, perhaps suggesting that he didn't have the necessary paperwork.