Act Party leader John Banks will make a rare vote against his coalition partner National this week to maintain his passionate defence of animal rights.
The Psychoactive Substances Bill is expected to return to Parliament tomorrow and the Government could come close to losing a vote on a Green Party amendment which would ban animal testing for the purposes of approving legal highs for sale.
The bill states that animal testing should not happen if alternatives exist, but some MPs want the tests ruled out.
A proposed ban on animal experiments tabled by Green MP Mojo Mathers has the support of Mr Banks, who told Parliament on Wednesday: "Protecting animals is ingrained in my soul."
Mr Banks said the bill was well-intentioned but he could not support it in its present form.
It was "totally unacceptable" that the bill failed to rule out "testing these recreational drugs on innocent animals".
When former Associate Health Minister Peter Dunne said in the House that it was never his intention to have animals experimented on, Mr Banks said this could not be guaranteed and described Mr Dunne as a "puppy-hater".
If it passed, Ms Mathers' amendment would block drug-makers from using information gained from animal testing to prove that their products were "low-risk".
It was opposed by National and United Future.
Whether it succeeds could hinge on the fact that new Labour MP Meka Whaitiri will not be sworn in when the vote is held.
At present, the amendment is believed to be supported by 60 MPs and opposed by 60 MPs. New Zealand First has not publicly stated its position but is expected to support the amendment.
It was expected to pass if Ms Whaitiri was able to cast her vote. But the new Ikaroa-Rawhiti MP, who was elected in a byelection last week, will not be sworn in until the end of the month.
If the vote is tied, the amendment fails.
Animal rights amendment
For: Labour, Greens, Mana, Maori, Act (53 votes)
Against: National, United Future (60 votes)
Undecided: NZ First, Brendan Horan (8 votes)