Update on the outcome. As I hadn't received an eTicket by Sunday 13 July (two weeks after submitting request via Fare Hold), I texted and phoned Air New Zealand contact centre. Was advised via text that the fare hold for DUD-AKL-DUD was still valid and eTicket would arrive within 48 hours. Still not showing last night, so sent another text. Received phone call this morning. I didn't end up using the credit as latest interpretation of US DOT rules means that we are now eligible for full refund for DUD-AKL-YVR returning on SFO-AKL-DUD (previously only SFO-AKL-DUD refund had been received, with balance in credit). I have now received the eTicket for DUD-AKL-DUD and email confirmation of the refund for DUD-AKL-YVR.