Besides, Abbott told Sunday's leaders' debate: “The GST can't change without the agreement of the states and territories.” Loading Replay Replay video Play video Play video But Rudd says the requirement for the approval of the states is set down in a piece of legislation Abbott can easily change. “Let me remind him about the agreement of the Commonwealth and states,” he said in the debate Sunday night. “That exists as a product of legislation by the Australian Parliament which any future parliament can change. Kevin Rudd's claim that you can change the GST without the consent of states and territories is true.

“Any future parliament” could adjust the adjust the GST as it liked. Could Rudd be right and all those previous assurances be wrong? Supporting evidence The assurances go back to 1999. Introducing the GST, treasurer Peter Costello told Parliament: "No change to the rate will be implemented without the unanimous support of all the states and territories and both houses of the Commonwealth Parliament. The bill clearly provides this lock-in mechanism." The law does indeed include such a lock-in mechanism.

But all of the constitutional experts PolitiFact has spoken to say it has force only until a new law is introduced removing it. An old parliament is unable to bind a new one. If it could, there would be not much point in elections. Here's how the 19th century British constitutional lawyer AV Dicey put it: “Parliament has the right to make or unmake any law whatever”. University of NSW constitutional expert George Williams says the only way a parliament could partially bind a future parliament would be to change the constitution. That's difficult, and it's only happened eight times. Sydney University expert Anne Twomey points out the Commonwealth also has a signed agreement with the states not to change the GST without their consent, but says it's “a political agreement, not a legally binding contract". Kate Carnell, who was a chief minister when the agreement was signed, has told PolitiFact she knew at the time “it wasn't legally enforceable".

How it stacks up Kevin Rudd is right about the law. A new government could change the GST as it liked. It would probably want political cover and so the agreement of the states has some political force, but if just some of the states wanted it and some held out, a newly elected Commonwealth government could go ahead anyway. Finding A PolitiFact rating of "true” applies where a statement is accurate and there is nothing significant missing. PolitiFact rates Kevin Rudd's claim about the GST “true”.

Details at www.politifact.com.au Loading Fairfax is partnering with the Pulitzer-prize winning service PolitiFact during the election campaign. Its Australian arm politifact.com.au uses the same rigorous methodology as its US parent to rate the accuracy of claims by elected officials and other influential people in the Australian political debate. Twitter: @1petermartin @PolitiFactOz