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Births are registered in NSW 
Eric F PLANE b. 1906 ALBURY #31610
Dorothy J PLANE b. 1906 ALBURY #31611
Parents: Thomas H & Mary
Deduced from NSW BDM online ..... Eric and Dorothy
Born 31 Dec 1906 
May I suggest if you are considering purchasing the birth certificates, that you order Official Transcriptions rather than the more expensive certificate. You will get the same information.

May I also suggest you get #31611 (Dorothy) as it is numbered next after Eric's. So, hopefully it will be for the younger of twins. NSW BDM birth registrations include details of the OLDER siblings .... so Dorothy's should include details for Eric.....

(two registrations for the price of one !) It should also include the name and relationship of the person registering the birth, and details about Thomas H and Mary
Here's the NSW BDM live link to their Official Transcription Agents.
http://www.bdm.nsw.gov.au/Pages/family-history/family-history.aspx#transcriptionWhat to expect on a NSW birth registration: 
Full given name/s, sex, date and place of birth (sometimes includes day of week, not just date and sometimes includes full street address/rural property name, not just town or village.)

Mum's details full name and maiden name (and perhaps any other former names), age, occupation, place of birth.

Dad's details - same depth of detail as for Mum .... so his full name, age, occupation, place of birth.

Date and place of parents' marriage.

Previous children of the relationship (name and age of living, number and gender of deceased).

Name and relationship of informant,

Name of Midwife/Doctor and any others present at the birth.
Cheers, JM