Neither of the two staff members involved in the crash were injured, but Kean said they were being provided with welfare support.
Ken earlier told The Herald there was no threat to public safety after the crash.
The accident will be subject to police investigation and WorkSafe has been notified, she said.
A police spokeswoman said police received reports at 4.10pm this afternoon of a van going off the road on SH1 between Chestnut Rd and Ohakuri Rd before crashing into a bank.
"Four people are injured and have a range of minor to moderate injuries."
The police spokeswoman said there were likely to be delays.
"There were two staff members and four prisoners in the vehicle. Police and ambulance are responding," she said.
A St John ambulance spokeswoman said the service was called to the crash at the same time as the police.
She said one person had been seriously injured while five others had been moderately injured.
She said an ambulance was on the scene, with another two ambulances and a helicopter on the way.
A Fire and Emergency NZ spokeswoman said the van had tipped on its side and crews were at the incident to manage scene safety.
The road was reopened by 9pm.
