Hi Sarah
From NSW BDM (free to search)
Marriage
6305/1891 CALDER WILLIAM S HORSLEY ADA M ORANGE
The marriage certificate should have the parents of William listed. (for a cheaper option hire a transcription agent-see NSW BDM)
Also here are some births to William and Ada
24984/1900 CALDER MARY E V father WILLIAM mother ADA M district ORANGE
26618/1908 CALDER JOSEPH B C WILLIAM ADA M MILLTHORPE
29571/1893 CALDER LESLIE W WILLIAM ADA M PETERSHAM
28724/1910 CALDER THOMAS M WILLIAM ADA M MILLTHORPE
24163/1898 CALDER JOHN E WILLIAM ADA M ORANGE
Also check out his tree
http://awtc.ancestry.com/cgi-bin/igm.cgi?op=GET&db=madde... It has William's death 31 Oct 1920
From Trove: Sydney Morning Herald Monday 1 November 1920 Page 8
CALDER-October 31, 1920. at his residence, Toongabbie, late of Orange, beloved husband of Ada Calder, aged 48 years. R.I.P
Then from the NSW BDM Deaths
20925/1920 CALDER WILLIAM LUKE ELLEN PARRAMATTA
Hope this helps
Ina