Mr Mitchell said this would impact baby boomers who would be tapping into the SuperGold Card scheme in the next few years, causing local and regional councils to be short of funds.
Mr Mitchell said the SuperGold Card allows holders to "connect with the community", and the changes would hit the "financially vulnerable".
However, Transport Minister Simon Bridges said the changes would not change entitlements for SuperGold Card holders at all.
"There is a lot of nonsense being talked about the SuperGold Card. Seniors have nothing to be concerned about. They'll be able to continue to use their service as they always have."
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Mr Bridges said they had taken the opportunity to wrap the SuperGold Card scheme into "bigger, broader changes" of bulk funding public services.
"We're doing that to get better value for money for the public transport service generally as well as for the SuperGold Card scheme," he said.
Bay of Plenty Regional Council transport policy manager Garry Maloney said the Ministry of Transport, the NZ Transport Agency and Local Government New Zealand, on behalf of all regional councils, were currently working on how the SuperGold Card scheme would work nationally and in the future.
Last week Local Government New Zealand said in a press release it welcomed the confirmation "that the entitlement and Government's support for those eligible for the SuperGold Card will not change".
President of Tauranga Grey Power Christina Humphreys said although the changes were "still a bit vague", if there were financial costs added for SuperGold Card users it would affect "a lot of people".
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She said many of those using the bus relied on the SuperGold Card concessions to be able to "live life", due to being under financial strain.
Ivan Schultz, an 85-year-old Taranaki man who was in Tauranga visiting his daughter, said he appreciated the free off-peak travel the SuperGold Card entitled him to.
"It's one of the only things I get out of it," he said.
He had just travelled from Bayfair back to the CBD, and said he was a regular user of the buses when he came to Tauranga.
The changes would also require card holders to purchase travel smartcards to provide operators, regional councils and government with data of the number and the length of trips taken.
Tauranga residents would have the Smartride Card, which is free of charge but needs $10 worth of credit to be added when purchased.
Regional councils would inform SuperGold Card holders of when they would need to obtain a smartcard for their area.
SuperGold Card:
* The SuperGold Card offers discounts and offers from businesses, government-funded free off-peak public transport and certain services and discounts from local councils across New Zealand.
* Off peak travel is between the hours of 9am and 3pm and from 6.30pm to the end of service on weekdays, and any time on weekends and public holidays.
* To be eligible for the SuperGold Card, users must be over 65-years-old and a resident of New Zealand, or under 65 and either receive the veteran's pension or the non-qualified spouse or partner rate of the NZ super or veteran's pension.