That is curious about Australia.... I thought that one of the international agreements regarding passports was an agreement whereby the holder of a valid passport could not be refused entry by the country that issued that passport.
No, there is no such international agreement or treaty that grants any rights to a citizen, acting on their own accord, to enter their home country. There are international agreements regarding deportation that guarantee a country the right to deport a passport holder back to their own country... but those agreements are facilitation agreements between countries, not any sort of guarantee of the rights of an individual.
I think that Australia's actions in this regard are unique and unprecedented. We'll see how it plays out... at present, all the Aussies inside the country are too worried about the spread of COVID to bother worrying about fellow citizens who can't get back in.
Things would get interesting if a person, arriving from a country where they were not a citizen, were refused entry into the next country.
If you pass border control when leaving but are denied entrance at the next, where do you stand? In many places there is quite some distance (measured in miles) between border controls.
It is very common for countries to refuse entry to non-citizens - this happens all the time, for all sorts of reasons.
In practice, a person has not "entered" a country until they have been granted entry by the border control officials of that country. This is a courtesy practice between adjacent border officials, not anything that is enshrined in international law. So if the receiving country doesn't want to accept the person, the country that they just left normally takes them back.
There are all sorts of exceptions to this, though. If you have two countries with adjoining borders who are openly hostile to each other, anything can happen. If a person claims refugee status when arriving at a country, that country is "generally" required to accept them
temporarily if the country is a signatory to the 1951
Refugee Convention and its
1967 Protocol. If a person sneaks into a country, the only way the country can get them out is to either find an adjoining country that is willing to accept them, or ask their country of citizenship to take them back.
Even here in Canada, have you officially entered the country once you step off the plane or do you have to go through customs first?
Except in the aforementioned case of refugees, a person is not considered to have "entered the country" when disembarking from a flight, ship, or vehicle until the immigration authorities have granted the person permission to enter the country.
Many times during my working career, when I landed in a country for a technical stop (refuelling), I never "entered the country" because the immigration officials couldn't be bothered to come out to the plane. They knew I was just there for refuelling and would be leaving in an hour or two - they had no interest in me at all.
Michael