Yesterday Ardern said she had previously made her intention clear to many ministers in Cabinet.
"That's been discussed before. We've had that conversation in Cabinet committee," Ardern said.
"At that point I made very clear it was my intention that we would have no other form of regional fuel tax [besides Auckland]."
She said Transport Minister Phil Twyford was part of that committee and should have been aware of her pledge.
She had not made an announcement about the policy earlier because "I hadn't been asked".
Twyford had implied that he first learned of the pledge on the morning that Ardern announced it, but later said that he knew about it from the start of the year.
Despite that, he did not inform any of the 14 councils that have expressed an interest in having a regional fuel tax.
"I've communicated to them very clearly that there will be no regional fuel taxes in this term of Government. The Prime Minister has taken that one step further."
Ardern also suggested Regional Economic Development Minister Shane Jones, who first learned of the pledge when Ardern announced it in the House, should have known earlier.
National's economic development spokesman Paul Goldsmith said Ardern's comments were "silly" and she should just admit to a U-turn.
"The Minister of Transport has always left the door open to future regional fuel taxes after 2021. That was the point of the legislation.
"It's a new policy she brought about on the hoof yesterday. There's nothing wrong with that. Governments should respond to public pressure, but she won't admit it, and that's the silly thing.
"She doesn't seem to be able to accept that she changed her mind, and is now blaming everybody else for her mistake."
Hamilton City Council had been eager to have a regional fuel tax, and Twyford previously encouraged it to engage with the Ministry of Transport.
Councillor Dave Macpherson, chairman the of the growth and infrastructure committee, said Ardern's pledge was like a kick in the guts.
"We're facing massive growth increases, particularly in the Hamilton-Auckland corridor, and we're putting in $230 million over the next 10 years on top of our normal transport budget to fund what we can. We're maxed out."
He said a 10c regional fuel tax made sense.
"It wasn't going to break the bank. We put up rates in July by 9.7 per cent. We think we're doing our bit, and more."
He said the Government would have to come up with the funds for the region's urgent transport needs.
