Ms Longstone began a five-year contact in November 2011, and left the role 15 months later on February 8 this year.
Her resignation was announced by Mr Rennie on December 20 last year.
He said they had reached a mutual agreement that she should resign, and he noted a strained relationship with Education Minister Hekia Parata.
Ms Longstone's resignation came after the ministry came under fire for Christchurch school closures, the botched Novopay teacher payment system, charter schools, closing residential schools and releasing national standards data.
Her payout, announced today, came on the same day members of the New Zealand Educational Institute, representing primary and intermediate teachers, protested outside the electorate offices of 35 National MPs over Novopay.
Mr Rennie said the troubles in education were the "tipping point" in an already strained relationship between Ms Longstone and Ms Parata.
The education minister would not comment today on Ms Longstone's payout.
In a two-line statement she said it was an employment issue and therefore a matter for the State Services Commission.
"I have acknowledged Ms Longstone's efforts and I wish her well.'
A spokeswoman for Mr Rennie said she would not confirm or deny whether Ms Longstone had been paid $50,000 to relocate to New Zealand, or if it would be paid back due to her early departure from her contract.
Ms Longstones's payout is comparatively high. Former Work and Income chief executive Janet Grossman, who similarly returned suddenly to Britain after only 11 months in the job, was paid $97,000.
Ms Grossman also received a one-off payment of $50,000 to relocate to New Zealand.
Labour and the Greens have slammed the payout.
Labour's education spokesman Chris Hipkins said the payout was outrageous.
"In reality, Lesley Longstone took the fall for Hekia Parata's incompetence, and National's unpopular policies. Taxpayers are paying a hefty price, just so that National had someone else to blame for the Government's stuff-ups in education.
"The Government claims the relationship between Lesley Longstone and Hekia Parata had broken down. Well, $425,000 dollars would have paid for a hell of a lot of relationship counselling".
Greens' co-leader Metiria Turei said the payout was s result of Ms Parata's inability to manage her education portfolio.
"Rather than let her work out her notice, which the State Services Commissioner said would have been normal, Hekia Parata's relationship with Ms Longstone was so dysfunctional that she had to be shown the door immediately, making the exit payment much higher than it should have been."
