Hi Isabel,
Can you please confirm if you are seeking info about Alison THOMSON, or about ALICE THOMSON or about Michael SHANNON...

There is a marriage registered in the Newtown district (of Sydney) New South Wales in 1933 for an Alice Thomson. NSW BDM registrations are usually full of family history info including for marriages:
For both the bride and the groom: full names (often including former, previous alternative etc), occupation, usual place of residence, conjugal status (bachelor/spinster, divorcee, widow/er), where and when she/he was born, sometimes even the exact date of birth, the father's name, sometimes occupation, and mother's maiden name (and if any parent is known to be deceased).
https://www.bdm.nsw.gov.au/Pages/family-history-research/registry-records.aspxNSW BDM has Official Transcription Agents, and they offer an excellent alternative to the 'real deal' certificates. I cannot tell from the online index IF this lass would be connected to your enquiry.
JM