Do you know that John was alive at his sister's wedding? If not, the only possible death I see is in Qld (not Brisbane) - John Edgar 5 Feb 1866 age given as 29, parents unknown (looking at Qld NSW Vic WA and Tas (only to 1899))
Not sure how much you have on the family in the UK. The detail from the immigration records gives the following - which confirms your Ann Ross for John's mother
Edgar Thomas 38 Plasterer born Dumphries, Dumphrieshire Religion - C of S read/write - both Ann 42 wife Kilmuir RosshireC of S both John H 15 Plasterer Carslile Cumberland C of S both Roslind A 5 daughter Westminister Mdx C of S neither Edgar Thomas 38 Plasterer Parish of St Marys, Dumphries, Co of Dumphrey Parents Thomas & Elizabeth mother living at kilmarnock, co of Ayreshire Ann's parents - David & Ann Ross both dead
This is the most likely index record for the birth of John GRO Index Birth Dec 1837EDGARJohn HendryCarlisle25 35
I have some Scottish family with the given name Hendry - often written as Henry in England/Australia. If this is the right birth, I would think John Harold is definitely not the right person.