It is unusual for Act, an historically libertarian party, to oppose a bill which will remove red tape for people wanting to develop land.
The bill will allow up to three homes of up to three storeys each to be built on most sites without the need for a costly and frustrating resource consent.
"I've got to go to caucus on these issues now," Seymour said.
Seymour said his opposition was on the grounds of principle and certainty.
"This is not so much deregulation, it is sweeping aside a system of rules that Auckland agreed to over a pretty tumultuous three or four years for the Auckland Unitary Plan, which already allows 420,000 additional dwellings," Seymour said.
Seymour disputed the idea that he was opposing the policy because of opposition to urban density from his leafy central Auckland seat of Epsom.
"That's a very cynical thing to say - my view is if we want to solve a very serious problem of housing affordability, those are real problems we need to solve.
"We've been thinking about this for years now, and planning law is part of the solution, but planning law alone is not going to solve this. If it did the AUP would have solved it years ago," he said.
National and Labour agreed to an historic truce on housing, agreeing to introduce and support the Housing Supply Bill which they say will see at least 48,200 and as many as 105,500 new homes built in the next five to eight years.
It is a significant political step for both parties, which face pressure from urban property owners who are unhappy with more housing being built in their neighbourhoods.
By both backing the bill, the parties have essentially agreed to not capitalise on that vote.
"Today is truly a historic moment for New Zealand: a time when our two major political parties stepped up together to give Kiwis the Right to Build," National leader Judith Collins said.
