You should obtain a Notice of Intended Marriage (NOIM) form from the person performing your the wedding ceremony in Australia or a copy can be found here. The NOIM must be signed in the presence of and witnessed by either:
a UK Notary Public (look in the Yellow Pages phone book under "Notaries")
an Australian Consular Officer at the Australian High Commission in London (fees apply for these services).
Once witnessed, the NOIM form should be sent to the Marriage Celebrant at least one month prior to the propsed wedding. The celebrant will advise you of any additional documents you may need to supply before the wedding can take place.