I am dissing that theory , too, on the grounds that he couldn't get back to his starting point, at least, by tram, he couldn't.
He would have missed the last tram back all the way to Melbourne City, and been stuck at Preston Depot.
Of course, he could follow a tram by car, but it seems she caught the tram ( at least, she was dropped off at Bourke St Mall) , no cars go up and down there, be hard for him to spot her , him driving up Elizabeth St,.. then , he has to find a place to turn round to go back... then, he has to follow that 86 from Lonsdale St, not Bourke, as it is closed off to car traffic …( he might have seen her from his car at a tramstop, along the route.. and followed the tram until he saw her get off )
Putting a bit of Occams razor to it, though, it has a tad more logic that he was either a late entry on that tram ( got ON not far from her stop, and saw her and got off at her stop ) or was already there, hanging around.
Maybe an habitual 'hanger around' that tramstop.