I still have my hunch that the NZ$493 million quoted in the original presser included a sustainment package. The RAN ships were $640m for two plus an additional sustainment package.
That seems highly likely for two reasons.
Firstly, the price (NZD$493/USD$350m) seems too high. See the RAN Navantia price above. An even better comparison, I think, is the Norwegian AOR being built by DSME to the Aegir 18R design that was also pitched to NZ/Aust. They are paying USDS$230 to DSME, plus unspecified amounts for locally fitted equipment. While not ice-strengthened, the ship is presumably winterised to a fair degree. While the ice strengthening undoubtedly increases costs, USD$120 (approx. 50% of purchase cost) seems a bit implausible.
Secondly, most recent complex purchases (Texans, Sea Sprites, simulators) have included a maintenance and support component in the purchase contract. If this is the direction the government wants to go, it seems more than likely they will use the same approach on the new tanker.
My guess is that the ship will come in well under USD$300 mil, and the rest of the money will be a support contract.