Megatron is perfectly legal in WA, but as Tober said, Customs is a Federal body, so they'll still confiscate the toy if they intercept it.

You need to obtain a Form B1709A, Importation of Firearms - Police and Certification permit from the WA Police Firearms Division. Once you've done that, Customs will release the toy to you (you might want to ask if this means them sending the toy to you, or if you have to go and physically collect it yourself from Customs (in which case you'll need to make time to do so)).

http://www.police.wa.gov.au/Ourservi...2/Default.aspx
http://megatron.net.au/MC_Masterpiece_MP05_WA.htm

Email: LicensingServicesFirearms@police.wa.gov.au
Postal Address: 297 Hay Street, East Perth WA 6004.
Phone: 1300 171 011
Fax: (08) 9223 7044