NZ Post says the volume of British passport applications is causing delays to them being sent to the UK.
A new requirement for dual nationality citizens to use a British passport to travel there took many travellers by surprise last month.
And with the deadline for the change only a fortnight away - 25 February - applications surged.
PostShop staff told customers they had to use an express service, which cost $121, to send their applications. But some people, following their packages' progress using online trackers, noticed the applications were being held up at an Auckland mail centre before being sent to the UK.
One customer said her package was waiting In Auckland for nearly two weeks, without anyone contacting her to say there was an issue.
The tracker now showed 'air transport planning is underway'.
NZ Post last week said incorrect customer declarations were responsible for delays.
But it has now apologised and said it was working to overcome the hold-up.
"Due to the increase in the number of passport applications being sent to the UK at the moment, it is taking a bit longer than usual for some of these items to move through their journey," said a spokesperson. "We're sorry for the delay and can assure customers that we are working as fast as possible to get these important items where they need to be."
Staff checked that each item had complete information on its customs declaration, she said.
"This is a manual process and is taking a bit longer than usual due to the increased number we are receiving. We can assure customers that we are working through these as quickly as possible, including contacting individual customers who have missed some information off their Customs Declarations. We're actively working on a solution to help speed this process up and get these items moving as fast as we can."
