There are British subjects living permanently in Australia, who are not Australian citizens, who are eligible to vote in federal elections and referendums.
Under section 93(1)(b)(ii) of the Commonwealth Electoral Act 1918, British subjects, who were enrolled for a federal electoral division in Australia immediately before 26 January 1984, are eligible to enrol and vote at federal elections and referendums. Section 4 of the Referendum (Machinery Provisions) Act 1984 also provides that electors who are entitled to vote at an election are entitled to vote at a referendum.
Eligibility to enrol and vote prior to 1984 was based on a combination of British subject status and a minimum Australian residence qualification of six months. In 1981, the Parliament passed legislative amendments changing the federal franchise qualifications. It was the view of all political parties and governments, state and federal, that Australian citizenship is the appropriate basis for the franchise. Parliaments in all states soon passed complementary legislation.
When the franchise qualifications were changed to require Australian citizenship, it was considered unfair to disenfranchise British subjects who were already enrolled and entitled to vote. Consequently, British subjects who were already enrolled at the date of transition were able to retain their franchise.
Yes, it is compulsory for British subjects who were enrolled for a federal electoral division in Australia immediately before 26 January 1984, to enrol to vote, even if following 25 January 1984 they have let their enrolment lapse. British subjects not on the roll immediately before 26 January 1984 are not eligible to enrol even if they were resident in Australia at that time.
Over time, the number of British subjects enrolled for a federal electoral division in Australia prior to 26 January 1984 is gradually diminishing.
Ireland is not a British Commonwealth country but is to be treated as such.
South Africa is currently (and was previously) a member of the Commonwealth, however, people born in South Africa are not eligible British subjects for the purpose of enrolment.