I'm using a 3G USB Stick for 2Degrees at the moment plugged into a Mikrotik Router.. I topped up a while ago and I used all my data up so it redirects to a page to tell me this so I can log in and add more money.
So I logged in (secure.2degreesmobile.co.nz) and went to the payment page and entered all the details but...
It seems virtual.paymark.co.nz is not available when you have no credit :-(
So I tethered to my iPhone and went to the same place and payed and went fine.
Also to note no where does it say you have to disconnect your 3G Connection and Connect again when you have started a new data Pack. (from having nothing)