I've just utilised my voucher(s) with no problems and no additional costs involved, the flight was actually cheaper than the voucher value by about $400 so will see if I get a new voucher for the balance. I called the local number in Auckland and they answered straight away, the only problem I have is that I have to hang around in Auckland until 4th June as there are no flights in May at all. I've had all sorts of hassle with SQ with my flights being cancelled 4 times and now being converted to an open ticket to be used before 30/03/2021 because they admitted that they would not be able to transport me in the foreseeable future and no tickets were available on their partner airlines either, I had booked the QR flight as a precaution as the SQ flights were looking dodgy but the QR flight also go cancelled hence the voucher.