If you are in contact with any of Ailsa's children they should be able to obtain her birth certificate.

Looks like the two marriages for your Ailsa.

Groom: Lawrence Harry MCKENZIE.
Bride: Ailsa Lynette LEAHY.
Year married: 1954.
Place: Tocumwal district, New South Wales.
Registration number: 4549/1954.

Groom: Gerald Charles CROUCHER.
Bride: Ailsa Lynette MCKENZIE.
Year married: 1966.
Place: Wollongong, New South Wales.
Registration number: 17574/1966.

Have you obtained any of the marriage certificates for Ailsa?
They may include a few details regarding Ailsa's parents names, her age when married and place of birth.

I haven’t obtained either certificate, I imagine the marriage to Lawrence is the most likely to have honest lineage on it, as she was younger. Only 17. How can I obtain a certificate? Can I get it online? Is it expensive?

Also, thanks for your help :)

hi James the only things i have found so far about her are newspaper articles which you may already have The Herald Mel. Vic Tuesday 8th June 1948 Girl Missing For 3 days and her engagement to Lawrence, it states that she is the only daughter of Mrs P Leahy of Melbourne
As I have been unable to find any birth reg for her I assume this was not her birth name
I will keep looking

Hi. I have an article where the it states the police are looking for her, but not the article where her mother is mentioned. Could you send the link to it, if it’s online? Thanks

Certificates cost $48.00 by post or $35.00 online and are available from the Registry of Births Deaths & Marriages in New South Wales.

You can also obtain a transcript of the certificate from a transcript agent for about $20.00.

Full details available on the BDM NSW site.

Gerard Charles Thomas (Gerry)CROUCHER died in Melbourne on the 15 December 1999, He was aged 60.

I could be wrong but I think Ailsa's mother was born in Shepparton to Michael Halpin and wife Ellen Keogh
I believe she married Patrick Francis Leahy in 1934.

Yes her name was Irene Edith Halpin, born around 1900

Could you provide me with some details as to why/how you come to this?
The Shepparton link is interesting because I have letters from a Mrs. Cleave writing to the orphanage where my dad was claiming to be Ailsa’s sister, she was living in Cosgrove outside of Shepparton.

Maybe this isn't the right family, there were quite a few P. Leahy's around But anyway, trial and error.
Name Irene Edith HALPIN
Spouse Patrick Francis LEAHY
Father Michael HALPIN
Mother Ellen KEOGH
Birth 1902 Shepparton Victoria Australia
Death 1972 Mooroopna, Victoria Australia
Patrick Francis LEAHY was born 21 October 1896 was the son of
Edmund LEAHY and Johanna ENGLISH
He and Irene married in 1934
He died at Shepparton 11 November 1970

Mrs P Leahy is stated as mother in the announcement of marriage the Lawrence McKenzie. So I’m not sure.. but do you have any further information?
It sure is a pain that Australia doesn’t have census info like England.. or I can’t find it atleast..

The Shepparton Family is not yours. They lived there all their lives (farmers) The girl went missing fromher home in Carlton.
We should be looking for Leahy's living in Carlton.

the engagement info i found was on trove Cobram Courier Family Notices Thursday 7th Jan 1954 Leahy - McKenzie
https://trove.nla.gov.au/newspaper/article/189890922?searchTerm=Ailsa%20Lynette%20Leahy&searchLimits=

Cobram Courier Thursday 7 January 1954
ENGAGEMENTS
Leahy - McKenzie
The engagement is announced
of Ailsa Lynette, only daughter
of Mrs. P Leahy, of Melbourne,
to Lawrence Harry, youngest son
of Mrs J. McKenzie, Wondah-Street, Cobram.

There is no mention of Mr Leahy just Mrs. Leahy from Melbourne. Which makes me think they were separated or he was dead; and when the girl was thirteen and roaming the streets her home was Carlton.
When I looked at the electoral rolls for Shepparton the Leahy couple up there, were together on the farm up to the 1968 roll, with, whom I assume was a son called. Peter Michael Leahy.

Ailsa Lynette Leahy
Father: Leonard Leahy
Mother: Evelyn Pearl Leahy nee Thompson
According to Ailsa's marriage certificate to Lawrence McKenzie

This could be the birth of Ailsa's mother.

Name: Evelyn Pearl THOMPSON.
Year born: 1915.
Place: Rutherglen, Victoria.
Father: Albert Edward THOMPSON.
Mother: Winifred Frances CHEGWEDDEN.

Source:
Registry of Births Victoria. Registration number 7250.

I saw that, it is the most likely candidate, but i can't find any connection to a Leonard Leahy..

Nothing located in the marriage records to date for LEAHY and THOMPSON.

There was a Pearl LEAHY died in Kew, Victoria, aged 57 years, but her parents were not named. She would have been born about 1915 going by age at death. She is buried in a lone grave at the Fawkner Memorial Park Cemetery.

Will follow the THOMPSON/CHEGWEDDEN line and see if I can find out what happened to this Evelyn Pearl.

I found this interesting snippet but unfortunately nothing else before or after.
The Herald Wed 21 Apr 1943 Page 3
Not Guilty Of Bigamy
Pearl Evelyn Leahy, of Johnston Street, Collingwood, was acquitted in the Criminal Court today by direction of the Chief Justice (Sir Frederick Mann) on a charge of bigamy. Mr J. T. Ryan appeared for the defence.

Interesting find by janilye about Pearl Not Guilty Of Bigamy.
Very strange. Does this indicate Pearl may have remarried under a different name?

Not sure if this marriage has anything to do with the above bigamy case but worth looking into.

Groom: Kenneth Charles FORSEY.
Bride: Pearl Evelyn THOMPSON.
Year married: 1941.
Place: Victoria.
Registration number: 19824.

Unable to confirm if this is the same Evelyn Pearl THOMPSON
born in Rutherglen in 1915 without marriage certificate.

Further to the above...

Kenneth Charles FORSEY remarried in 1946 and died in Richmond Victoria, aged 28 years.

The way I see it is Kenneth and Pearl must have divorced between the years 1941 and 1946.

Is it possible this bigamy charge against Pearl in 1943 had something to do with the divorce?

This is only my opinion of course.