An investment regime would be a natural extension of the TPP, but it strikes some critics as a threat to national sovereignty. It is not, though our Trade Negotiations Minister, Tim Groser, took an oddly different view when interviewed for the Weekend Herald. "Of course trade negotiations involve concessions over the sovereign rights of countries to do things," he said. New Zealand had suffered from the "excessive sovereignty" of countries that subsidised or protected their farmers. "The whole point of international law is to put limits around countries' sovereignty ..."
Someone in Mr Groser's position can be forgiven for exaggerating the force of trade agreements. Whatever may be agreed, it will rest always on the voluntary adherence of sovereign members. If a future New Zealand government was to dishonour an investment code agreed under the TPP, there is nothing the other signatories could do about it. New Zealand would pay a price in its diminished attraction for investors but its sovereignty would not be at risk.
Fears of an investment regime are being propagated widely. Even a group of state legislators in the US last week expressed concern to President Obama's Trade Representative, Ron Kirk.
International tribunals normally hear disputes between governments and this proposal would let investors sue governments directly. But investors already have that right in domestic courts and it is hard to see that an international court would be more "chilling" for governments.
Those not in the habit of confiscating property or destroying wealth without compensation would have no reason to fear investors' rights, and those with such inclinations would probably not be deterred. The best trade negotiators can do is strike a fair deal in good faith between nations that can be trusted.
Not many nations promote free trade for its inherent economic benefits. Too many still treat it as a concession to be bargained. By making common cause with true free traders, New Zealand seems to be getting somewhere.
Mr Groser, a trade negotiator long before he became the minister, is aiming for a "gold standard" extension of the TPP but, crucially, he is prepared to accept failure.
"We don't need a second-rate agreement with anyone," he says.
New Zealanders will hold him to that.