Nerf guns aren't imitation/replica guns. Other toy guns are required to have a bright orange cap on the end of the barrel so they can't reasonably taken to be a firearm. If a sci-fi prop laser gun has no real wold equivalent and can't reasonably be taken to be a firearm it should be okay to own or import. (Although, if you're still uncertain about a specific item you're planning to import, it's probably best to contact customs or your relevant police department.)

the issue with replica and imitation firearms is that they can be used to exert fear. You can't really do that with a nerf gun.

Customs (only) definition of a Prohibited "Imitation" (section 4F of Regulations 1956) -

Quote:
"imitation" means an article, of any material or colour:
(a) that:
(i) is a copy or reproduction of a firearm; or
(ii) has the appearance of a firearm; and
(b) that is not capable of discharging shot, bullets or other projectiles by means of an explosive charge or a compressed gas; and
(c) that could reasonably be taken to be a firearm; and
(d) that is not a blank-fire firearm.
As for customs, I'm pretty sure they just X-ray stuff to see what's in it and they'll open a package if they think it's suspicious.