^any amount, if there is supporting evidence (eg deal bags, scales, wads of cash etc). Or you could do what that poor footy player did and say "it's not for me" (= trafficking).
Else it's 3g (I think - you could look up drugs poisons and controlled substances act - or you could utfse here on bl)
And purity doesn't count - it could be pure talc and you could still be charged if police feel it was being presented as a drug.