For one way ticket on temporary visa, check if your visa label has a line that says something like "no return ticket is required". If there isn't a line like that, do check with INZ whether you'd get into trouble in the check-in counter - ask them to either replace your label to include that line or write you a letter saying it's ok for you to have a one-way ticket.
When we were on work visa, my partner had that line and somehow I didn't. I kept getting into trouble checking in (twice!). The check-in officer in Singapore called Auckland INZ to verify! I had to explain to the INZ officer that I'm aware my visa is expiring soon but I was also applying for Residence; AND I will renew my work visa before it expires if Residence application were still under review then.