Yep, I see that argument and how it might feel like a new tax.
But, GST was always supposed to be paid on those sub $400 items. They just haven't been collecting it.
I think most people will perceive this as a new tax. And perceptions count at elections!
It is a new tax, because the tax is imposed on foreign businesses where previously it was not. The fact that residents had a theoretical tax liability which was not realised, is immaterial.