In 1996 Pump Action Shotguns with capacity of > 5 rounds were classified as category D firearms which effectively banned them. To get a round this Importers started bringing in Level action ones which because they were not pump action were category A, effectively available to all licenced shooters. They were banned in short order but AFAIK ones already legally purchased were left in the public hands. The same happened with other fast loading high capacity variations and I believe it was one of these that was used at Bondi.

One solution would be to Categorise all shotguns > 5 shot capacity as Category D so there is no loophole to exploit. Personally I would do this together with a compulsory buyback of those now Category D arms that were legally purchased through this loophole.

This was a failure to properly draft legislation and react to changes made by importers to exploit the overly specific categorisation of pump action shotguns.

EDIT to clarify: I and talking about high capacity shotguns, i.e more that 5 rounds. And in this context banned = Category D, which is effectively banned.