Unfortunately airsoft is currently illegal in Australia, but what part of an airsoft gun is actually illegal? Is it the fact that it fires a projectile at high speeds? The fact that they look like real weapons? Both? Neither? Something else? Additionally, what distinguishes airsoft from things like Nerf blasters, which are allowed?
Wikipedia is pretty comprehensive.
As a general rule, the following types of airsoft guns are illegal in all states:
Guns capable of fully automatic fire.
Guns that outwardly resemble a sub-machine gun or machine pistol.
In addition, specific states have further rules e.g. in NSW they are strictly prohibited, whereas in the NT they are otherwise allowed if you have the right licence.
As to why, that is a question for politics stack exchange.