A republic in and of itself it not necessarily a good thing. Not PC akins it to accepting a kidney transplant from a bureaucrat, but I think it is more like a trojan horse. It looks like something good, but you don't know what's inside, or why you got it. The motivations of some advocating a republic should be cause for worry.
You see a republic can range from being a constitutionally limited one, that is meant to constrain the role of the state, like the United States, or it may be a corrupt dictatorship, like the Republic of Tajikistan. I don't expect Keith Locke wants a "People's Republic" although he has been cheerleader for this in the past, but I also don't expect he wants to emulate the United States.
So whilst a debate on this is good, indeed very good, be wary of those who push a republic for the sake of a republic. If a republic appears in the coming years, it is a once in a lifetime chance to fundamentally change the constitutional structure of New Zealand and ringfence the role of the state - and equally to constitutionally demand an expansion or entrenchment of it.
Have a guess to what extent Keith Locke wants to constrain the role of the state, and to what extent he wants to expand and entrench it.
Then ask yourself if you really think that those who will advance a republic will predominantly share that view, or will they advance a republic should tightly define the state as an entity to protect individual rights and freedoms.
I doubt it is the latter, and as a result, whilst I would advocate for the latter, I'd prefer the status quo to any vision of a republic Keith Locke has.