What is a double dissolution election?

The governor-general, on the advice of the prime minister, has the power to dissolve the House of Representatives and the Senate and call an election. The point of the double dissolution election is that it allows the Australian people to break a deadlock in Parliament caused by particularly contentious legislation.

How is it triggered?

Both chambers need to disagree on at least one piece of legislation. This can occur when the government of the day - holding a majority in the House - does not have a majority in the Senate. The bill needs to pass the House and fail in the Senate on two occasions, more than three months apart. The Governor-General can, under section 5 of the constitution, appoint times to hold sessions of the Parliament "as he thinks fit".

Prime Minister Malcolm Turnbull announced on Monday he had received permission from the Governor-General to recall both houses of parliament on April 18 to reconsider the government's key union reforms, which have been stalled in the Senate since last August, and could form triggers for a double dissolution election.

How does it work?

When the governor-general dissolves the Parliament, all members and senators face a general election. Once the new composition of the chambers is decided, the bill that caused the disagreement is again presented to the Parliament.

The option for a double-dissolution election is available until May 2016, as they cannot occur within the last six months of an ordinary three year term of the House of Representatives.

What triggers does the current government have?

If the Senate fails to pass either the ABCC Bill or the Registered Organisations bill when Parliament is recalled in April, Mr Turnbull will advise the Governor-General to dissolve both houses and call the election for July 2.The Senate has once rejected the ABCC Bill, which aims to reinstate the Howard-era Australian Building and Construction Commission - abolished under Labor and twice rejected the Registered Organisations Bill - to create an independent union regulator with stricter penalties on union officials. The latter is most likely to form the trigger for a double dissolution election.Mr Turnbull said that the bills would ensure that "unions are more accountable, more transparent, managed in the same manner, transparent manner that public companies should be managed."

It is possible for a government to stockpile triggers for a double dissolution if they wish to get them all passed in one go.

While the government has other failed legislation, including a bill to abolish the Clean Energy Finance Corporation, one to establish the Asset Recycling Fund and another to abolish the Australian National Preventative Health Agency - Mr Turnbull made clear that they would only call a double dissolution election if either of the industrial relations reforms failed to pass.

Has it happened before?

There have been six examples in Australian history. Between 1914 and 1987, the government of the day lost those elections on three occasions. The most famous double dissolution election occurred in 1975 when one was called by newly installed caretaker prime minister Malcolm Fraser after the dismissal of the Whitlam government.

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