Ms Bennett this morning told Radio New Zealand she rejected Ms King's allegations.
She claimed her letter was a response to a much wider payment card proposal than the one announced at the National Party conference on Sunday.
"How ridiculous saying it is a backtrack. It's not a backtrack at all...
"I was writing a response to one correspondent who is proposing something very different."
Despite that, Ms Bennett said she could not rule out extending the payment card system across the benefit system.
"I can't rule anything out at this stage because that would be doing announcements which are yet to come."
She said teenage beneficiaries were an exceptional case that justified more Government intervention in their financial decisions.
Thousands of young people were falling through the cracks and need more guidance, she said. "We pretty much give them hundreds of dollars a week and leave them to it and hope they'll be okay."
The letter, written by Ms Bennett in March, said the writer's suggestion for a special credit card system for beneficiaries would require the Government to make "moral judgements about the appropriateness of each decision''.
"Such oversight by the Crown would be highly intrusive and would rob individuals of their freedom of choice,'' Ms Bennett wrote.
"Spread across the entire benefit system, this kind of oversight would impose an enormous administrative burden and cost upon Work and Income.''
Questioned in Parliament about the letter by Ms King yesterday, Ms Bennett said the current policy was for 16, 17, and some 18-year-olds, not all beneficiaries.
"I have said that I do actually see it as intrusive. I do think the administration that comes with it is worth it, and I am backing these young people into a better life.''
Ms King said the letter exposed National's policy as a political rort.
''...three months out from an election we suddenly have Paula Bennett singing the praises of a policy that just five months ago she dismissed as something that was intrusive and would impinge on freedom of choice.
"I believe the minister owes the country, and especially those 58,000 young people facing what, under National, would be a very bleak future, an explanation,'' Ms King said.