"We don't know if people have forgotten or if they have left the country or they're deceased or too frail to drive - there's a whole lot of caveats around it," he said.
It's estimated about 29,000 people die each year in New Zealand - only a fraction of our three million motorists.
Acting road policing manager Inspector Peter McKennie said drivers without a current licence risk a $400 fine and a ban from driving. "Yes, there are people driving without a driver licence when police stop them. Yes, there's a fine but there's also another consequence - they get forbidden to drive so they're not allowed to drive any motor vehicle until they get their licence renewed.
"If they get caught driving a motor vehicle before they've got their licence renewed then they run the risk of further prosecution and having their car impounded," he said.
Police can use discretion when someone has genuinely forgotten to renew a licence, but they will be banned from driving until it is updated.
McKennie said there was a proportion of motorists who refused to update their licences. "They're probably more of a concern for us. Some of those drivers have got no real intention to go and get it updated, and some of the people are perhaps a risk to other road users."
Insurance Council of NZ chief executive Tim Grafton was surprised at the sheer number of expired licences.
"You put yourself at risk, not only are you breaking the law but that any insurance claims you may make whilst as a result of driving might be declined."
The Herald on Sunday spoke with people in the streets this week and several were surprised to discover their licences had expired, or were about to. Aucklander Jane Mountjoy was stunned to find her licence expired in a matter of weeks.
"It's a good job you've reminded me. I remembered thinking it was ages away and here we are - and it isn't," said the 55-year-old grandmother.
Knackstedt said the transport agency sent out reminder letters when licences were up for renewal. When photo licences were introduced from 1999, renewals were staggered over the first decade. With the phase-in cycle complete, renewal is now once every 10 years.
If a licence is expired for more than five years drivers must re-sit parts of the exam.