Booking international travel when you have dual-citizenship

My partner Steph, our kids and I are going on holiday from New Zealand to the United Kingdom and Europe in early 2026. We hit some snags this time with changes to how customs works in the UK.

The difficulty comes from Steph and I being dual citizens of New Zealand and the UK. Steph was born in the UK and emigrated to New Zealand as a child. I am a New Zealand born and managed to get UK citizenship because my father was born there.

Previously when travelling internationally, we have each used a single passport for the whole journey, generally whichever was still valid and did not require renewing. Most people have only one passport, so using one passport is the common case when booking flights.

We booked flights with an online discount booking agent. Passport details were required to complete the booking, we used our still New Zealand passports, thinking we would be travelling with these on all legs of the journey.

It turns out that you now need an ETA, Electronic Travel Authorisation, to enter the UK on a New Zealand passport. Previously this was not the case. The kicker was that from February 2026, if you are a UK citizen you must now use your UK passport to enter and leave the UK, you cannot get an ETA on a foreign passport.

What does mean for our flight booking? When travelling with more than one passport, which passport should one use when booking the flights? With these questions, using an online discount booking agent was less than ideal. The booking agent, for what appears to be a combination of cost saving and legal reasons, was practically allergic to giving advice about what passport should be used on the booking, or whether different passports could be provided for separate flights in the same booking.

With, flights already booked and paid for, and an unhelpful booking agent, I had to fall back to "internet research." Here is what I think are the correct answers, which I would of hoped the booking agent could have given me:

The short answer is, in the booking, enter the passport you intend to use when exiting the first country, prior to your first international flight.

And the long answer is:

• You can only put one passport on a booking, don't let that confuse you, what really matters is what happens when you enter and leave each country. 
• You must always enter and leave a country with the same passport, otherwise you will appear to have overstayed, which will likely cause you difficulties the next time you visit that country.
• Many countries, e.g. Australia, and the UK from 2026, will not allow you to enter or leave using a foreign passport if you are a citizen, i.e. you must use a passport issued by that country if you can get one.
• Always have on your person all your passports, including the one you made the booking with, when you go through customs. If there is a problem, produce all the passports and the customs officer will pick the right/best passport to use. 
• If you are entering or leaving a country on a passport other than the passport you made the booking, don't be surprised if you cannot go through the fast lanes at customs. 
• If you are a dual citizen, trying to save money by not renewing all your passports is not always possible, and when possible may be a false economy.

As I say, I am pretty sure this is correct. I'll post an update if/when we get home.