HIGH COURT CHALLENGE TO SAME-SEX MARRIAGE POSTAL VOTE:
* Three people, independent MP Andrew Wilkie and marriage equality advocates Shelley Argent and Felicity Marlowe say the postal ballot breaches the constitution.
* An injunction is being sought to head off the ballot, which is due to start in mid-September.
* The government is relying on section 51 of the constitution, which says the federal parliament has the power to make laws with respect to "census and statistics". The relevant laws are the Census and Statistics Act and the ABS Act. The Treasurer will direct the Australian Statistician to request "statistical information" from all Australians on the electoral roll as to their views on allowing same-sex couples to marry.
* The precedent is a 1974 Whitlam government phone survey of 60,000 randomly selected Australians through the ABS as to their view on the national anthem.
* Those behind the court challenge say the statistics power in the constitution can't be relied on for the postal ballot.
* The Finance Minister says he can appropriate the money for the ABS through an advance available to him of up to $295 million if there is an "urgent need for expenditure". And the ABS can second officers from the Australian Electoral Commission to assist.
* The challengers say there is no demonstrated "urgency" and the money cannot be spent without legislation.