You should be charged GST as it is over the limit, however it seems common practice for all grey retailers to undervalue the item. It is fraud and Customs are well within their power to hold the item and give you a heavy fine. Whether this happens on a regular basis is probably unlikely, but I know someone who has been nabbed trying to import a 5DII + 300 2.8.
Not trying to scare you but it can and occasionally does happen if Customs suspect the individual to be skirting the law.
I think buying grey is a real personal choice depending on who you talk to. Over on Whirlpool there are countless threads of bad experiences and some good ones with DWI. It's a matter of balancing the risk - I'm quite happy to buy a grey Nikon SB-910 as the savings are considerable and for the $400 if it does cark it well not a big deal for that amount of money. However, maybe not with precision optics such as lenses and cameras.. indeed I managed to get a better deal from JB buying the Nikon 24-70 2.8 for $1680 and the D800 for $3350. At the time, greys were equal or higher than JB when postage was included.
I guess, however, if I were to buy from a grey retailer I'd try and buy from one who is Australian based with a registered business. That way despite the manufacturer's warranty (or lack of) you still have 12 months under Australian Consumer Law. There's a fellow in Perth that comes to mind, however I'm still deciding what I want to get