Hi S Yep that's the issue. Nord VPN. Forgot that it autoconnects whenever I turn my PC on. I've disconnected and can log in now. Thanks Rebecca

@Yvonne you are a life saver! Trademe kept erroring for me saying I needed to fix my pick up address. I tried over and over again to fix it but the problem persisted, even when copying and pasting directly from the NZ Post website. After reading your comment about checking the address being sent to, I checked it and it turned out the postcode was wrong for that address. So thank you! Life would have been so much easier if Trademe's error indicated it was the destination address that was the issue and not the pick up address.