Geoffers, the age is correct (court records state that he was 20 in 1834) & I know of 2 other Wm Mumfords in Tasmania at the same time. However my man was on the Augusta Jessie (confirmed by convict number).
"Folio 1: William Mumford, aged 20; sick or hurt, ulcus; put on sick list 16 June 1835, discharged 25 June 1835"
Thanks for the search though.