Hi Val, It has been a long time since you posted this message so I hope that this reaches you.
Have you considered (or even realized) that Henry was very likely to be the man who married in NSW in 1873: 902/1873HENRY JOHN AUGE ISABELLAGUNNERY SYDNEY 902/1873HENRY JOAN ONGE ROSABELLA GUNNERY SYDNEY
I know that this registration doesn't look like him but when you match it with the marriage announcement from Trove, you get the name H. Irvine ONGE. It would be very interesting to find out the witnesses and the parents and may give you some more clues. I suspect that if you can identify his parents that you will have a very interesting history. I think that he hasn't been entirely honest somewhere along the line.
I have been on the trail of Rosabell but I'm not a descendant.