Has anyone had recent experience in how to extract a GST invoice from NZ Telecom for a Pre-pay XT on-line topup?

I paid a $60 top-up by credit card, but the confirmation is not a GST invoice, it merely quotes the total amount and does not quote GST.

NZ IRD requires a GST invoice for payments over $50 and unfortunately I chose a $60 top-up.

There is an absolute obligation in NZ GST law on the supplier (Telecom) to provide a GST invoice within 28 days of a request. I have 'phoned Telecom 123 and after about half-an-hour's discussion the CSO offered merely Telecom's GST registration number and although the CSO had gone off-line a couple of times to consult higher authority, no provision of a GST invoice has been offered nor any procedure to follow to obtain one. There has been a promise to e-mail me a further response, but given the lack of any sign of getting to grips with the matter so far, I don't have much hope.

I should rather avoid the paper-work of having to write to the Commissioner of Inland Revenue about this, so if anyone knows a less-onerous way of obtaining such an invoice I should be grateful.