Australian High Commission
United Kingdom
Australia House, London

Cert of No Imped

If you are an Australian citizen or permanent resident wishing to marry overseas, the government of the country you are marrying in may request that you have a Certificate of No Impediment to Marriage (CNI) issued. 
You should clarify with them who can issue the CNI for you. Many foreign governments will only accept CNIs issued by the Australian mission (embassy or consulate) within their country.
A copy of the CNI form can be downloaded here.  If you choose to complete the application by hand, print clearly and in CAPITALS or it may be issued incorrectly.
Instructions on having your CNI witnessed can be found within that application.
The High Commission can produce your CNI either by post, or in person during notarial hours.
There are fees associated with obtaining a CNI from the High Commission