I found some (slightly conflicting) info via Google...
L. Stone, portrait of a young lad of about 6 years or so, photographed by Stewart & Co., 217 Bourke Street East, Melbourne, who operated there between 1879 and 1891
Stewart & Co. 217 Bourke St. East, Melbourne (Farmer Album - Willie 1874)
McLaughlin, bust portrait of a young lady aged about twenty five or thirty; photographed by Stewart & Co., of Bourke Street East, Melbourne, who operated in that vicinity in the 1880's.
[Edward John Dunn]. 35 x 50 mm. A carte de visite head and shoulders studio portrait of Edward Dunn, probably taken around 1870, shortly before he left Australia. The backmark reads: 'Stewart and Co, 217 Bourke Street East, near Parliament Houses, Melbourne. A large framed copy of this portrait may be had price 20. Marion, Imp, Paris'.
Photographer: Stewart & Co, 217 & 219 Bourke Street East, Melbourne, Australia.
Date: Signed on back on 24th July 1880.
An interesting snippet: Artist Tom Roberts was employed as a photographer's assistant at Stewart & Co in the 1870s.http://www.artistsfootsteps.com/html/Roberts_biography.htm