An AGS number is a government service number for Australians. You will not have an AGS number unless you are currently employed in the government. If you do not already have this number and are an employee of Australian government, your recruiting agency will issue you one. If you do have the number but you don't know where to look for it, there are two simple ways of finding it.
Look for AGS number on your pay slip. Look for an "Employee ID" number on the pay slip. This is the AGS number you're looking for.
Contact the HR department of the agency where you were last employed.
Ask the HR department to look into their records to find your AGS number. You will be asked for your full name and name of last position held with the agency.