I have also bought from
DWI...A Sigma 10-20 lens. No problems with them or the lens at all. Shipping was swift. The lens arrived in 5 days. I would have liked to bought local, but the savings by buying from HK (approx $150) made all the difference. I wouldn't hesitate to buy from them again.
Maybe I am a bit naive , but the way I look at is that the chances of anything going wrong with a lens in the warranty period (assuming it is in perfect working condition in the first place and that you
look after it properly is pretty remote. So when it comes to a lens I don't particularly worry about the warranty being honoured in AU (a camera body would probably be a different story though).
I admit to never having thought about it being fake/remanufactured or whatever, but I would imagine that if you stick to a reputable seller your chances would be slim of getting one like that.