Optional preferential voting
Optional Preferential Voting (OPV) is a system of vote-casting used in New South Wales and Northern Territory elections in the Commonwealth of Australia.[1][2] All other Australian lower house elections are run under full-preferential voting, where all of the candidates must be ranked in order of the preference of the voter to be counted.
Under OPV, voters may choose to mark a preference for one candidate (effectively voting as though it were a first-past-the-post election), all candidates or any number of candidates in between. Although complete numbering is not required under the OPV system (effectively a full-preference vote), single-preference voters are supposed to use a '1' rather than a tick or cross. In practice, ticks or crosses are still considered valid votes as the intention is clear.
In 2016, Senate Voting Reform in Australia switched the Senate ballot papers from running on full-preference voting to OPV.
In other countries, such as Malta, the Republic of Ireland, and Northern Ireland, full preferences are not required.
OPV allows for one single candidate or candidates of similar ideology to be endorsed by a voter rather than directing preferences to all candidates.