I am going to make a speculative suggestion here as to Stephen's motivation.
Selina was expecting, or thought she was expecting a baby .
There is not a birth record that I can locate, but as we sadly know, not all pregnancies reach full term.
Or
The couple already had an exnuptial child.
Stephen was not prepared to take on married life, but elected matrimony to make and honest women of Selina. There may have been family pressure from either side.
As soon as the paperwork was completed, he disappeared.
A transcription of the marriage certificate may reveal something, such as a shared residential address.
One of the many reliable agents for transcription.
http://www.joymurrin.com.au/shop/nsw-transcription/ What you can see on a marriage certificate NSW.
MARRIAGE
Date
Couple's names
Couple's ages
Couple's birthplaces
Previous status
Couple's occupations
Couple's residences
Fathers' names
Fathers' occupations
Groom's mothers name
Bride's mothers name
Sue