Labour MP David Parker says he held out hope National would support a Bill that would ban conversion therapy right up until Thursday's vote.
National was the only party to vote against Justice Minister Kris Faafoi's Conversion Practices Prohibition Legislation Bill, which easily passed with support from ACT, the Greens, the Māori Party and Labour, which holds an absolute majority on its own.
Earlier this year National leader Judith Collins said the party didn't hold a position on conversion therapy, the pseudoscientific practice of changing or suppressing a person's sexual orientation, gender identity, or gender expression. Former leader Simon Bridges said in February he was concerned banning it would be an attack on free speech and "cancel culture".
After Collins looked it up on Google, she came out against it - and the rest of the party fell into line.
Bridges told The AM Show on Friday they're all for a ban, but won't vote for the current Bill for one reason.
"We support the intent fully, we wish we could get behind it. We have one major concern and by the way Kris Faafoi, the guy behind it, can't and won't explain it at any level. I've looked through the law really closely and it comes down to this - the Bill will criminalise, as it's written, good parents for being parents.
"If you've got a 12-year-old who's going through change and so on and you say 'look actually, tai ho son/daughter, we don't want you to go on puberty blockers, to have the various medical treatments, until you're 18', you will be a criminal facing up to five years in prison, and that's wrong."
By 18 most, if not all, teenagers would have been through puberty and already developed secondary sex characteristics. Puberty blockers delay its onset, giving kids who haven't yet figured out their identity yet more time to understand where they sit on the spectrum. Supporters say they're fully reversible - as soon as their use is stopped, puberty will continue as normal.
Bridges says puberty blockers pose risks parents might not be aware of.
"The experience offshore - if you take the UK, what the courts over there are now saying are these medical treatments are innovative and experimental and there are long-term risks and consequences. We want a parental exemption, and if we get that actually we will be fully on board."
The Young Nats on Thursday urged their elders to back the Bill at its first reading.
"I understand the good place the Young Nats are coming from, but I'm not in the business of criminalising, the National Party's not in the business of criminalising parents for being parents," said Bridges.
Parker, appearing on The AM Show with Bridges, said he wasn't sure which way National would vote.
"I wasn't sure because there's geen a number of positions expressed by the National Party. So, I did hope they'd support it at first reading."
National MP Chris Bishop, seen as being from the liberal wing of the party, said on Twitter he remained "confident Parliament can improve the law so we can support the Bill into law". Other users of the site said he was being "disingenuous", pointing out that by voting against the Bill at its first reading, National was effectively trying to stop it going to select committee, where changes can be made.
"If only there were a process for reading a bill a first time, sending it to a select committee to work on, returning the reworked bill for a second reading," one person responded.
"You can improve it in committee. You voted to *not* improve it but instead to throw it out entirely. You know this," said another.
"You voted against sending it to select committee, you gigantic hypocritical tool," said a third.
Bishop's tweet got what Twitter users call 'ratioed' - when a tweet gets many more critical replies than it retweets and likes, showing widespread disapproval.
Parker played down fears parents would face prosecution, saying such action wouldn't be common - requiring the authority of the attorney-general.
"Isn't the greatest ill here the terrible harm that can be done to young people struggling with their sexuality being forced by anyone really?"