Kyanar:

 

You may find an NZ citizen endorsement worth doing. That way can you can enter NZ on your UK passport, without having to faff about with multiple passports.

 

I did consider this but the endorsement is the same price as a new 10 year passport so I opted for the NZ passport. The passport is more useful to as transitting via Australia has become more difficult and now requires an E-visa for UK passports if you are there for more than a few hours. A few recent trips to the US have been via Sydney with 5-6 hour lay overs.

Kyanar:

 

You can actually apply for a new ESTA with your new NZ passport, during the application process it asks for all your passports and links all of them meaning you should be able to travel to the US on whatever passport you please

 

Well...it depends. After a dozen or so business trips to the USA over the past 20 years I had a run in with an over zealous CBP agent in SFO. I was denied entry as he was adamant I was there for work - at the time I was a sole trader which made it difficult to prove my position (eg they cant just call your boss).

Denied entry means no more ESTA - forever supposedly.

The Auckland consulate didnt have any issues giving me a 10 year business visa after explaining what happened.

I have heard on other travel forums of people getting ESTA again after a few years (but I dont know if they did this by omiting certain information).

I did apply for an ESTA but was unceremoniously denied (after they took my money :D ) which wasnt unexpected but it was worth a try (BTW I said I had been denied entry before and added both passports to the form).

Anyway, I have several years left on the UK passport but I'll likley apply for a new visa in the NZ passport to simplify things. Unfortunately that isnt cheap if outside Auckland as you have to attend in person to get a visa.