Ever wondered why you can't smile in a passport photo? Or what the chip contains?

All this and more is answered by Maria Robertson, deputy chief executive of service delivery and operations at the Department of Internal Affairs.

If you smile, it changes the structure of your face, she tells RNZ's Afternoons.

"It's more about your bone structure in your face. When you smile, you smile differently every time and when you're at rest, you're basically at rest."

The imagery for facial biometric match relied on 17 different points on your face, so resting face worked better, she said.

Most rejected passport applications were because of the photos, she said.

"I was truly astonished at some of the photos people were sending in."

One woman took a selfie on a plane, she said.

"With the phone sitting on her lap, looking up towards her chin and her face.

"It was rejected, and she replaces it with a pic of her head-to-toe standing on the bow of a yacht."

The chip contained all the information about you that was in the physical document, she said, but the physical part was also pretty complex.

"The hard page in your passport has seven layers of polycarbonate.

"Sealed in there is an antennae, a copper wire, when you put the passport into the machine at the border that puts power into the passport, links up the antennae and sends that data to an international passport database that checks its validity."

Counterfeiting a New Zealand passport would be a tall order, she told Afternoons.

"You'd have to be able to print yourself high-quality secure papers with special inks, you'd have to be able to use the right type of hinge to bind the passport together.

"The right type of polycarbonate, the right chip and infrastructure to bring the number to life."

The risk now was deep fakes, she said.

"People using technology to try and game the system.

"We have to make sure the information we are getting from people is an input to confirming their identity is authentic."

And yes, people did Photoshop pictures to make themselves look hotter, she said.

"We get all of that. People who take photos of other photos, people who use filters, people who send children's school photos."

And selfies did not cut it, she said.

"The dimensions of your face are all changed."

Finally, if you haven't signed your passport, it is not valid, and technically not your passport.

"It belongs to the Crown, it's issued to you, but it's on loan, it's the property of the New Zealand government."

-RNZ.