Why doesn't Australia have any bombers?
Modern long range bomber is an offensive weapon.
Australia have no actual enemy that can reasonably threaten them. Papua New Guinea and New Zealand are an ally. Indonesia is a friendly business partner and a neutral. There is nobody in the South.
If Australia have significant amount of offensive weapon such as ballistic missile and long range bomber without any actual threat, the neutral nations to the North (such as Indonesia who have a fairly fanatical nationalism and anti imperialism) will feel suspicious of Australia's intention.
This can trigger unnecessary arm race, military build up, bilateral dispute, and Indonesia leaning to (in the past) USSR and (today) China.
We have 72 x F18s, 24 x Super Hornets, 2 x F35s, 18 x P8s, and 26 x Orions, all capable of dropping bombs.
Having 142 airplanes capable of dropping bombs means we don't really need any specialised single purpose planes devoted to this role. Particularly as there's not a whole lot of places around here to bomb.