You have to be covered by a valid visa at all times to be legal in NZ. Since your present visa is linked to your current employment, if you ended that employment, you would have made yourself illegal, and would be obliged to leave as soon as you could get a flight.
https://www.immigration.govt.nz/new-...r-visa-expires As you will see at that link, if you were to get another job offer while illegal and before you had left the country, there is an outside chance that you MIGHT get another visa under Section 61, but an application in those circumstances is not on the usual terms and there is no obligation on the Minister to grant it, so it's really something to be avoided.
Or would I be entitled to look for another job and apply for VOC if I can find a prospective employer?
Yes.
As this link
https://www.immigration.govt.nz/new-...at-will-happen, given before, says, there's a chance of being allowed a Dismissed Worker visa if the termination happened within a trial period. If that doesn't apply, there have been times in the past when INZ has held off from cancelling the visa if the person had another job offer lined up that they could get a VOC or another visa for, but that isn't automatic, and quite honestly, I think it's less likely to happen these days, with the politicians' strong push to give jobs to NZers now the job market has shrunk so much due to Covid-19.
Look at my posts #4 and #5 above. You are in a much more secure position visa-wise if you stay in your present employment. If you can possibly sort out your work situation, perhaps by getting some advice or mentoring so as to be able to fulfil your employers' expectations, that is your best bet for securing your future in NZ. Otherwise, if you could get the offer of a comparable job with another employer, you could liaise with INZ to get a VOC or another visa (depending on the details), but it would be important that one visa should neatly take over from the other at the time you changed to the new job, and INZ would help with that. You can't work for a new employer while your visa names the original one, or continue with the old employer when once you have a visa that names the new one.