why australia want to sign international treaties?

Answer 1

So they can participate for mutual benefits.

Answer 2

Hey, recent treaties or in the past?
Australia joined ANZUS in the hope that the US and NZ would come to its aid if a threat arose. Other treates are SEATO and the Colombo Treaty.
Mostly Australia signs treaties so that helping other countries will result in a return of resources/aid/troops etc
For more info on Australian treaties try - http://www.info.dfat.gov.au/treaties
hope that helps! :)