FWIW, I've made aprx 18 nights of bookings between now and Christmas in the last week. I have not been able to pay online at the Mercure Dunedin, but have had no issues at the Ibis Styles Invercargill using a corporate AMEX.
I was told upon checkin that there are apparently 6 properties worldwide that don't allow online payment via AMEX, but this sounds incorrect reading through posts here.....
They were happy for me to ring and give details over the phone at the online advertised rate.