Police Clearance Certificates are often required by employers as well as for migration purposes, and it has come to my attention that some people have stated that a Police Clearance Certificate (PCC) is valid for 2 years.
In most cases, including for migration purposes, a PCC is only valid for 12 months. Some employers may choose to allow a longer validity term for their own purposes.
A statement on the Western Australian Police website, gives a pretty clear idea of the PCC.
Q: How long is a National Police Certificate valid?
A: A National Police Certificate lists the disclosable criminal history information that is available on the day it is issued. WA Police imposes no period of validity on NPCs; it is up to employers and organisations to determine the currency of a NPC they will accept.
Information on Police Clearance Certificates can be obtained in the UK from: www.acpo.police.uk
Migration Purposes:
Everyone who wishes to enter Australia is required to be of good character. In order for the Australian Government to determine whether you are of good character, you may be asked to provide a police certificate. Unless otherwise advised by the Department of Immigration and Citizenship (the department), police certificates are not required for persons under the age of 16 years.
http://www.immi.gov.au/allforms/character-requirements/character-penal.pdf