If you have your “own” driver’s license, it does not mean that you are legal to drive. Similarly, you can have and issue your own CoC, however subject to the requirements for issuing a PIRB CoC, a PIRB CoC must still be included and issued on all plumbing work undertaken by a Licenced plumber which includes but is not limited to geysers, solar water installations, heat pumps, etc. Furthermore, there are no real substances to a plumber having their own CoC. If this is the case they would be acting as the “Judge, Jury and Hangman”.