Its wear in the shifter its self. When you go out of the reverse gate the gear lever moves but the selector stays where it was. If you then stick it in first you are now in two gears at once. There are washers and bushes that become loose over time allowing this to happen. But if you get excited when you pull it out of reverse it will happen anyway.
Seems holden converted what was basically a column shift setup to floor shift, and they were on a tight budget.
Finding second gear in an aussie 4 speed when it was cold was an issue from new from what I have heard. It will go in, but it clearly doesn't want to. Perhaps thats why most aussie 4 speeds blew up. I skip second and go for 3rd when its cold.
You can see why everyone went for a smooth shifting 5 speed conversion in the day.
How come no one ever made a quality after market shifter for the Aussie 4 speed? Reckon there would have been a big market for one in the 70's/80's.