First appearance in Victoria appears to be with birth of a daughter named Florence DUGAS in Footscray in 1891 (reg 33131). Parents Lawrence and Ann (nee VENN). Sadly, Florence died in 1912 in Carlton, aged 19 (reg 939).
Lawrence and Ann lost another daughter at age 19, in 1905 - Beatrice, in Williamstown (reg 7394).
Then marriage of Jno Jas DUGAS, born England, to Annie COMERFORD in 1901 as you already know (reg 6029). John died in Melbourne in 1940 aged 63, as DUCAS (reg 12495). See
http://www.australiancemeteries.com/vic/maribymong/footscray...Jno Jas and Annie had the following children:
Margaret Ann b. 1903, m. Roy Alexander CRICHTON 1929, d. 1977 (m. & d. as Mary Ann)
Beatrice b. 1905, d. 1905 (days old)
Violet Florence b. 1906, m. Ronald WILES 1929, d. 1974
James Laurence/Lawrence b. 1909, m. Margaret Elizabeth PEATLING 1937, d. 1959 (m. & d. reg as DUCAS)
Rose Margaret b. 1912, m. Leslie Jno RICHARDS 1932, .d ?
The other daughter of Lawrence, Margaret DUGAS, born India, married in 1901 to Jno Wm KNOTT (reg 4040). They had the following children, all born in Footscray:
Myrtle Margaret b. 1902
Jno Arthur b. 1904, d. 1970 (as Jno Arthur Wm)
Phyllis Irene b. 1911
Allen Roy b. 1915, d. 1959 (as Allan Roy)
Margaret died in 1917 in Carlton, aged only 36 (reg 7816), as Margaret Ann KNOTT - death reg confirms parents names. Burial in Footscray:
http://www.australiancemeteries.com/vic/maribymong/footscray...Margaret being born in India fits with Lawrence serving in the British forces.
P.S. If you search FreeBMD, you will find the birth of John James DUGAS in 1877 in Farnham. This certificate might give you the clues you need.
P.P.S. There are 4 phone listings in Victoria for the surname DUCAS. Assuming John Lawrence and Margaret Elizabeth (nee PEATLING) had sons, this might be worth checking out -
http://www.whitepages.com.au/#