I think you should hasten slowly, and work back from one of the birth certs for a child of Mary Ann Amelia ROELS ... The official transcription will have all the same info as on a real deal cert. So it will give you first hand information likely provided by Mary A A ROELS about not just the birth being registered but about herself...
"Births :
Full name, sex, date and place of birth
Mother's details including
full name and maiden name, age, occupation, place of birth
Father's details including full name, age, occupation, place of birth
Date and place of parent's marriagePrevious children of the relationship"
http://www.bdm.nsw.gov.au/bdm_fh/bdm_rec.html
Likely births for Mary A and Phillip J ROELS/ROLLS/ROALES (I may have missed some, sorry )
1860 Maria #182 (ROLLS on index)
1862 Ellen E # 484
1864 Florence A # 632
1865 Blanche M , # 2434
1868 Sydney FA #4446
1870 Philip Henry G # 5108 and 187002456 045/1870 (two reference numbers, so there was something causing concerns to Reg Gen)
1873 Laurence Peter McPherson #3950
1878 Thomas (ROALES) #4647 (death 1878 #2864)
1878 Jacoba #5045 (death 1878 #3048)
If (as seems likely) it turns out that Hannah Sophia’s mum was the Mrs Roels of Camden St, NEWTOWN, and if you have lots of pennies …. Then if you want Hannah’s younger half siblings details :
I think that if you were to order an official transcription of the 1878 Jacoba birth cert you would have quality info to follow up for validating “known” info ….. a) when/where Mary Ann and Philip married, b) their ages in 1878, c) Dad’s occupation d) the family’s usual address e) midwife (could be a female family member, living nearby) and of course f) details of Jacoba’s older siblings…. You should also get Mary Ann’s birth surname, and her place of birth (same for Philip) .... Of course, it depends on who was registering the births, and how clearly the Deputy registrar heard the info 
Cheers, JM
Cheers, JM