Short answer: the stop was probably lawful; the green sticker is more contestable but often still lawful under current NZ transport law. The details matter a lot though.

🚓 1. Was the stop lawful?

Under the Land Transport Act 1998, Police have very broad powers to stop vehicles.

They can stop a vehicle:

• At any time, for enforcement purposes
• Without needing suspicion

That includes checking:

• Registration
• Licensing (rego)
• Warrant of Fitness (WoF)
• Driver compliance

👉 So even if the reason was:

“We recognised this vehicle as a trust-owned vehicle with non-standard plates”

That still falls within:

• Vehicle compliance enforcement
• Identification of vehicles not properly registered

✔ Conclusion:
The stop itself was almost certainly lawful.

🚫 2. The registration issue (this is the key problem)

In New Zealand, to be used on public roads a vehicle must be:

• Registered with Waka Kotahi NZ Transport Agency
• Display official number plates
• Have a valid WoF (or CoF for applicable vehicles)

👉 A private trust registration system is not recognised in NZ law for road use.

So even if:

• The vehicle is genuinely owned by a trust
• It is mechanically roadworthy
• It has a “Trust Certificate of Fitness”

That does not satisfy statutory requirements.

✔ Legally, the vehicle would be treated as:

• Unregistered
• Unlicensed
• Potentially unfit (in a legal sense)

🟢 3. Was the green sticker lawful?

A green sticker (notice of prohibition) is used when a vehicle is:

• Unsafe
• Or not compliant with legal requirements

Police can issue this if they believe the vehicle:

• Poses a risk to safety OR
• Is sufficiently non-compliant to justify removal from the road

⚖️ The tricky part

You’re raising an important distinction:

“The car was actually roadworthy”

That may be true mechanically — but:

👉 NZ law separates:

• Mechanical safety
• Legal compliance

Even if mechanically sound, a vehicle can still be:

• Legally unsafe to operate
• Because it is unregistered / uncertified under NZ law

✔ Likely Police justification

Police would argue:

• No valid WoF → cannot verify safety under NZ system
• Not registered → not traceable / accountable
• Non-standard plates → enforcement risk

👉 That combination is usually enough to justify a green sticker.

⚠️ Where you could challenge it

A green sticker may be challengeable if:

• The officer did not actually assess safety risk
• They issued it purely because they disagreed with the trust system
• The decision was unreasonable or disproportionate

🧠 Key legal issue in your situation

This is the core conflict:

Your position:

• Vehicle is roadworthy
• Owned under a private trust
• Certified under a parallel system

Legal reality:

• NZ law does not recognise alternative certification systems
• Compliance must be through statutory regime

⚖️ NZBORA angle (possible but difficult)

You could try to argue under the New Zealand Bill of Rights Act 1990:

• Arbitrary detention (s22)
• Unreasonable seizure (s21)

BUT courts have consistently held:

• Vehicle regulation is a justified limitation
• Road safety enforcement is a strong state interest

👉 So NZBORA arguments here are not strong on their own

🧾 Bottom line

✔ The stop:

Lawful — Police can stop any vehicle for compliance

⚠️ The green sticker:

Likely lawful, because:

• Vehicle was not registered with NZTA
• No valid WoF under NZ law
• Non-standard plates

Even if:

• The vehicle was mechanically sound
• You had a trust-based certification

👉 NZ law doesn’t currently recognise that system.

🧠 Where your strongest argument actually is

Not:

• “The car was safe”

But:

• Was the officer’s decision to green sticker proportionate and properly reasoned?

That’s a much narrower — but more realistic — line of challenge.