Mr Johnstone - who moved to New Zealand four-and-a-half years ago and lives in the Auckland suburb of Mission Bay - said the Scottish side have a good foundation and their Kiwi coach Vern Cotter "has really put something solid in place".
"Give it a couple of weeks, when the World Cup's over and you get into the new year and Six Nations come along. The team and the fans should be rightly optimistic."
He was still "gutted" by yesterday's loss, however.
"When it all dies down, the genuine fans will look at the game and feel a bit aggrieved by it, but then start to recognise that they gave a very good account of themselves," he said.
His friends in Scotland were "fairly upset" or "sort of emotionally distraught" by the result, he said.
He, like many others, was confused and annoyed by the match-winning penalty awarded to Australia by referee Craig Joubert in the closing minutes, but said it was one decision in 80 minutes of play.
"I think there's just an element of frustration with quite a bit of the refereeing or TMO work that's gone on during the World Cup. I know there's been quite a bit of inconsistency there."
But at the end of the day, Mr Johnstone said, Scotland were outplayed by the Australians - "if we're honestly going to look at the 80 minutes".