See my post # 2 about James serving with the Scots Gurards. His military record is available online.
I can assure you that the Scots Guard NEVER served in Australia during WW1.
James was demobilized on 4 April 1919. He was then a civilian and would have been unable to travel as a military service man. At that time, farm workers were in demand, and he would have returned to his farm work.
If you have found a Private J Forsyth inbound to Adelaide in 1919, (although you have no record ! ), it will most probably be an Australian soldier returning after the war. For the reasons above, it is quite improbable that it is your Scotsman James.