The thing is, Australian law does recognise dual citizenship and has ever since the late 1940s when Australian citizenship first came into existence. Australia does not require a person to renounce foreign citizenships and it doesn’t happen automatically on becoming an Australian citizen.
You can lose a foreign citizenship if that country’s own laws extinguish your citizenship on becoming an Australian citizen. But Australian law will not deprive you of it.
You can renounce it - but the problem in all these cases is for various reasons, most of the people were unaware they even had a foreign citizenship. A couple of them probably should have been aware (if you were born overseas the possibility should certainly have occurred to you) but for the others - it’s more a matter of other countries giving out citizenship very easily to descendants of people from their countries.
One problem with the gentleman whose father had connections to NZ is that both NZers and Aussies could just go back and forth without a passport...and work in the other country...
It was just like going to another state, here...
The fact that he didn’t know is feasible...