There is no way that the bad-guy, Stone, would ever be able to get his hands on the explosives he required for the vest-bomb he made. This is doubly more difficult for someone like Stone, who is obviously not the sharpest tool in the shed.
Explosives like firearms, are heavily regulated in Australia. In the extreme, incredibly unlikely event of a person robbing a bank followed by holding hostages in a nearby café, by use of a bomb-vest, were to occur, it would trigger an immediate national investigation. Political parties would be instantly under fire for allowing such a heinous event to take place under their watch. And new legislation would forthwith come under effect regulating explosives even further.
The point is, unlike seemingly in the USA, if something like this was to happen in Australia, it would just not be another weekday.
An investigation and federal inquiry would be in effect to work out how these explosives were able to get out into the wild.
Australian laws, Australian Federal-politicians responsibilities, and Australian public attitudes to guns and violence are not American ones.
Indeed, none of the NCIS, nor AFP, nor onsite police personnel, would just walk away from this, without every decision and action having been thoroughly scrutinized. Even at the cost of dissolving the NCIS-AFP task force.
Even Stone (and Louie, a 15-year-old kid) actually having access to pistols would be a near impossibility.
Robbing a bank and holding hostages by the wearing of a bomb-vest by the perpetrator, does not happen in Australia. And if it were to unconscionably happen, the consequences would be a lot more extensive than the attitude of the NCIS Sydney's team (in fact, of every USA police procedural's attitude} of just another day, just another case.