Can anyone tell me if you need a import permit to bring in a muzzle brake from the U.S ? i have been told you dont .Thanks

They're not included as a 'firearm part' or otherwise in the Customs Prohibited Imports Regs. So unless there's some Defence Dept requirement, no.
Factsheet: http://www.customs.gov.au/site/page4369.asp#e1057

May be a problem at the other end though, if over US$100 - gets into US export license territory. US$250+ in fees, and 4-6 weeks delay.

Thankyou Doug cheers