Well Sarah, when I buy things online from overseas, they don't add anything to the advertised price except shipping. I do believe that if you spend more than a certain amount, you will get charged duty coming in, which may be what you mean? I think it is 19%. I've bought to the value of around $300 NZ and never hit it, myself, but I have heard it happen to others in that price range.
So, if you keep your purchases fairly small, you should be fine!
Things bought from NZ online shops attract GST which is 12.5% - prices here include it usually but people ordering from them from overseas don't pay it so they add it later. I don't buy clothes online within Nz, but lots of other things from jams and chutneys to fabric.