Two vacancies still remain.
The NZTA also has yet to replace its chief executive, with former Chorus CEO Mark Ratcliffe serving as temporary chief since the resignation of Fergus Gammie in December.
The new members include former Maritime New Zealand chief Catherine Taylor, Rintoul Group director Ken Rintoul, Chartered Accountants president Cassandra Crowley, transport blogger Patrick Reynolds and professional director Victoria Carter.
Asked why he had made the sweeping changes, Twyford replied: "What we're doing is cleaning up the mess that National left after nine years of NZTA under National's leadership.
"... Not doing its regulatory job properly, not keeping New Zealanders safe on the roads ... and a completely unbalanced transport policy that spent 40 per cent of the budget on new motorways at the expense of every other part of the transport system."
But National Party transport spokesman Chris Bishop said the problems with the NZTA were at government policy level, not with the boards.
He said Twyford had also now had three chairs, after the resignation of Michael Stiassny in April after a shake-up at the agency.
"How can a mass clean-out of the NZTA board improve this disastrous situation? There will be a mass loss of institutional expertise and knowledge," Bishop said.
"Mr Twyford needs to realise his policies are the problem, not the board members."
Twyford denied he was churning through people.
He said the appointment of a new chief executive was "very advanced".