I've since had a read of some of the terms and conditions on East Coast's website (pasted below). All of the following notwithstanding, given that I contracted with Sixt for service, how would I have known about or be expected to know about any of this third party's policies beforehand is still unclear, especially when their existence as part of this transaction was at best poorly communicated and at worst not disclosed at all. In any case, perhaps they were running the card with the wrong account type selected on their terminals. That seems like the best explanation I can think of, but I'm not sure why three representatives couldn't have figured that out. Is it my fault for assuming they had prior experience running a point of sale terminal? Probably.

From their website:

Holding Deposit
East Coast Car Rentals will pre-authorise an amount up to the chosen excess on the hirers credit card. The pre-authorisation amount will be held by your credit card issuing bank and will automatically be released in approximately 5-7 business days. Debit Cards and International issued Credit Cards may take longer.

Hirers not using the credit card nominated for security as their payment method will automatically have the premium reduction waiver added to their rental at the prevailing rate.

The use of Debit Card as security will incur a 5% surcharge on the full cost of rental.

Credit Card payments will incur a Merchant Service Fee of 3.0% in addition to the total amount payable.

http://www.eastcoastcarrentals.co.nz...nd-conditions/