g'day, Bob. This is such a tragic song. Joe I always imagined it was about farewelling mates going off to the Great War. it was in a collection that was first published 1918.
But a re-read of the lyrics shows Lawson hasn't mentioned the war. Whatever the reason for the exodus of young men sailing out of Australia, the song is a plaintive celebration of mateship and times gone by. The song is about a choice, a choice to go away or to stay. as the lyrics say..

"But I'll try my luck for a cheque Out Back,
Then a last good-bye to the bush;
For my heart's away on the Outside track,
On the track of the steerage push."

This says to me that the Outside Track is the track of the steamship, taking young men away (to war or wherever).

The exodus of young people from the bush, whether overseas or to the cities for jobs, is still lamented because there's a history in the bush that is so different from the experiences of people in suburban and urban Australia.

freda