Thomas, a talented artist who studied sculpture with Charles Summers in Melbourne in the 1860s, painted his oil portrait in 1866, and donated it to the Melbourne collection after his death. She also wrote an adulatory biography of Summers: A Hero of the Workshop (1879). She worked mostly in England after 1867.
For Thomas, see http://adb.anu.edu.au/biography/thomas-margaret-4707 (biography by Marjorie Tipping, published in ADB vol.6, 1976). See also http://en.wikipedia.org/wiki/Margaret_Thomas#cite_note-ReferenceA-1 (noting a recent suggestion that her date of birth was actually 1842)