They both come up on the NSW BDM's that are online...they cover deaths up to 1981 and in the Ryerson Index.
If you have any problem locating them, email me on email@example.com
and I'll send you the direct links later today when we get home. With the Ryerson Index, only the bare details are given but you can request a look-up of the actual notice. It's a great web site.
I had been thinking that, since Melville passed away in 1955, Mary Margaret Spence may have re-married and her death notice may also appear but under another surname. However the online marriages for NSW only go up to 1961 and she hadn't remarried up to that time.