She said Segways were also banned from public roads in Britain and Australia.
James Law, principal of the agency, said he thought Segways were "closer to being a mobility device" and was "very surprised" to learn that they are considered motor vehicles.
"At a maximum speed of 18km/h, our agents could get killed operating them on the road if they are banned from footpaths."
Segway NZ director Philip Bendall said he was standing by the advice he gave to the firm that it was legal for them to use the machines to get around the central city.
"Our view is that Segway does not need to be registered because the personal transporter ... meets the definition of a mobility device."
He said there were more than 300 Segways in New Zealand, and about 150 were being used by people with mobility impairment or other disabilities.
"My advice to James and his agents is to carry on operating the Segways safely and respectfully under the operating rules of a mobility device. That's what New Zealanders have been doing since 2003 without causing any problems to anybody.
"This piece of law has never been tested in court to date, and we stand behind our view, and based on the evidence we've got, it meets the ... mobility device definition."
Segways are also being used for takeaway and pizza delivery, and during the Rugby World Cup "rugby ambassadors" used them to promote the event in the central city.
Last week Segways were used at the Lantern Festival to patrol Albert Park. Auckland Council spokeswoman Angela Jones said the Segways were used off-road and only in "parks and streets closed off" for the event.
She said the Segways were hired via a third party, and the "council does not use them and has no plans to".
The Ministry of Transport will be reviewing the status of Segways, but said it was not a high priority.