I'm assuming that your visa is the Post Study Work Visa (employer assisted).
https://www.immigration.govt.nz/new-...loyer-assisted
You can only ever get granted this visa once UNLESS you do another NZ qualification that entitles you in between times. On that link, it says plainly:
You can be granted this visa more than once, but you’ll need to complete another acceptable qualification each time you apply.
Whenever you are in a situation that you would have enough points for residence under SMC, you can apply for it, as long as, at the time, you are on a valid visa. So yes, if you get a promotion and a pay rise, that might make the difference. But the amount of salary you will need isn't just $20 an hour. See here.
https://www.immigration.govt.nz/opsmanual/66788.htm SM 6.10.5. Your present job is a Skill Level 4.
http://www.abs.gov.au/ausstats/abs@....il+Supervisors If you become a store manager, that is Skill Level 2.
If you don't get to a higher rate of pay, and you are approaching the time when your present visa will run out, then you would have to think what to do next. You'd have to think, would the present job qualify you for an Essential Skills visa? - and at the present rate of pay, it wouldn't.
http://skillshortages.immigration.go...il-supervisor/ If you weren't in line for any promotion and pay rise, you might need to look for a different job with better prospects.