Hi Kaytee,
I have a large document of the two JOHNSTON families of the 1880's, of Yass. Thomas JOHNSTON, and his brother William JOHNSTON.
The only JOHNSTON girl registered on NSW BDM's 1886, at Yass was Rosabell, known as Maud. Not sure if this is the mother. She married 1908, at Yass. Not known if she ever lived in Wagga, but her cousins did; the children of Thomas and Mary Ann JOHNSTON.
6560/1908 HARRIS WALTER L JOHNSTON ROSABEL C M YASS
There could be a possibility, that Thomas and Mary Ann JOHNSTON, had a daughter named Marie/Mary JOHNSTON in about 1886, which is not showing on the NSW BDM's, as not all BDM's, are on the database.
One way to find out, would be to purchase the birth certificate, of a younger child, such as Linda Thelma JOHNSTON (below), 1891, which should list the children, born before her.
35378/1891 JOHNSTON LINDA T THOMAS MARY A WAGGA WAGGA
Mary