Write to the board with this as your defence:
Provide a copy of the CP's timesheet showing his time on the job and secondly provide yours showing you did not work there. Assuming they don't accept that, this will work:
You were not guilty of a breach of a current enactment as they say. G12/AS1 is a non mandatory document that can be used to show compliance but is not the law. They have sighted this document as the 'enactment' you are in breach of.
Clause G12.3.8, which is mandatory, basically requires the HWC to be able to relieve excessive pressure(part a of G12.3.
and limit temperatures to avoid flash steam production(G12.3.8 part b).
All there is to prove is that when the TPR was opened that the relief drain at its lesser grade allowed for the water to drain away faster than the KW rating of the heater could heat at. If it backed up and could not relieve the water fast enough then it does not comply with G12. If it works they have absolutely no case.
As an aside the work you've done to remedy the situation is more of an issue. PVC is not suitable for a relief drain and it should be to a readily visible location, not to a downpipe. Here's a blurb the DBH did following a few faults:http://www.dbh.govt.nz/codewords-10-article-4
A bit on the non mandatory status of G12/AS1:http://www.dbh.govt.nz/building-about-compliance
Taken from that:
The Compliance Documents are not mandatory
, but they are important because they are published and endorsed by the Department of Building and Housing
If you need a hand writing a letter I'd be happy to help, I love helping out the wrongfully accused.