RYA accreditation

I thought all NZC’s advanced sailing courses are effectively RYA courses. And that NZ-era are now eligible to apply for European ICCs. So it sounds like you should have everything you need for a direct issue.

The only caveat with that is that Hugh did his NZ C qualification way back in 2001.
Coast Guard have likely lifted their game since then.

I wouldn't be surprised if Hugh will need to re-do a qualification to make sure it's relevant. Possibly easiest just to do the CG's ICC course. I would say the first step is to confirm with CG Boating Education for cross over from 2001.

Just had a quick look at Coastguard Boating Education to remind myself.

The ICC requires a practical component. The Boatmaster does not, therefore I doubt CG Boatmaster is any good for accreditation to ICC.
Note that I am assuming when Hugh said he did 'Yachtmaster' with CG in 2001, he is referring to the Boatmaster certificate, 30hrs of classroom time plus a written and oral assessment. I did mine at about the same time (early 2000's). It was about the only CG qualification available back in the day.

Having a quick look at available courses for ICC in Auckland, they are all on water sailing school providers, reinforcing the practical component. As I said previously, I believe the ICC's main purpose is to make sure you can get a charter boat out of and back into a berth. I have heard previously that all people had to do was show they could launch a run-about off a trailer, do a lap of the marina and get it back onto the trailer at the boat ramp.

ICC (International Certificate of Competance) - What is it?
The ICC is a certificate, issued according to Resolution 40 of the United
Nations Economic Commission for Europe (UNECE) Inland Transport
committee, and intended to provide evidence of competence to operate a
particular vessel when requested by officials in foreign countries.
It is an assurance from one government to another that the certificate
holder is sufficiently competent to be driving a pleasure craft, despite not
holding the visited country’s national certificate. The ICC should be
automatically accepted in countries which have adopted Resolution 40,
however not all of the UNECE countries have fully adopted it.
The onus is on ICC holders to determine its acceptability by foreign states,
as the ICC was never intended to be an alternative to individual national
qualification requirements.
Who is eligible for an ICC?
The ICC is now available to New Zealand citizens. Nationals of many
other countries are also eligible, including Australia, South Africa, UK
and USA. Citizens of other European countries will usually need to
apply through their own national authority.

How do I get an ICC and how much does it cost?
ICCs are issued following a short practical and knowledge assessment at an RYA training Centre or by
supplying another certificate as evidence of competence. RYA Training Centres will charge for their time and use
of a training vessel (if required) to conduct the assessment, and any training you may require beforehand.
ICCs incur a admin/processing/certificate fee of $95.00 - payable to CBE.
Holders of certificates accepted as evidence of competence can apply direct to CBE. All applicant requiring an
assessment should apply through an RYA Training Centre.

www.boatingeducation.org.nz/courses/

I'm not following your question now.
Do you have an RYA Yachtmaster that you got via Coastguard Boating Education?

If this is the case, it easily covers ICC.
RYA dictate the syllabus, there are loads of certified training organisations globally, normally sailing schools, but Coastguard Boating Education is one. So even if you did the qual via NZ Coastguard, it is still an RYA qualification.

Noting there is a big difference between CG Boatmaster and RYA Yachtmaster (hence my earlier comments)

Probably not.
I mean, you'd have to do an update module on how to navigate without charts, a compass or a sextant.
Was that before auxiliary engines were invented? You might need to do a module on diesel engine maintenance and operation
And a module on what modern yacht materials are, assuming 1976 was carvel planked wood and hemp rope, you'd need to get up to speed on how to splice a UhMPE-LGBQ+ brace
Oh, and you'd definitely have to do a module on diversity and inclusion, and crew mental well-being. You just can't chuck a drunken sailor in a longboat till he's sober anymore...

I have never heard of a CG Yachtmaster.
It is not listed under the CG Boating Education qualifications.

There is Boatmaster, which is a theory course and wont accredit to ICC.
There is the RYA Yachtmaster that definitely covers ICC.

Short story, if there was a practical component to your yachtmaster I'd say it will accredit to ICC. So if you did an on water assessment or on water training with an instructor I think you'd be fine.
If it was only theory it wont.