You  will need to supply a cert to say that the appliance is safe to use, cheers

Found it - Gas (Safety and Measurement) Regulations 2010 - Part 5 - Safety of Gas Installations and Certification of gasfitting work... (in part)

Part 44
Safety of gas installations
(1)  Every person who commissions, installs, or tests a gas installation or a part ofa gas installation, or an extension or addition to, or replacement of, as gas installation or part of a gas installation, must commission, install and test that gas installation or part, or the extension, addition, or replacement, as the case may be, in accordance with NZS 5621
(note this is now 5601

There are 6 parts to this section... then:

44  What types of gasfitting must be certified
(1)  For the purposes of section 47(1) of the Act, this regulation and regulations 46 to 48 apply to the following kinds of gasfitting:
(a)  gasfitting on newly constructed or important gas installations:
(b) extensions and additions to, and replacements of, existing gas installations:
(c) alternations to gas installations that result in repositioning of pipework or changes to the operation of the installation:
(d)  repairs to gas installations, gas appliances, or fittings following accidents that are notifiable under section 17 of the Act

(2)  Every person who is responsible for carryin gout any gasfitting to which this regulation applies, or, if that person is acting under supervision, the person who is responsible for that supervision, must ensure that the gasfitting is certified by a person who is authorised under the Plumbers, Gasfittes and Drainlayers Act 2006 to certify that kind of gasfitting.

(3)  Gasfitting that is carried out under an employer licence issued under sction 62 of the PGD Act 2006 may be certified by persons authorised to certify under that system of operation.

(4)  Gasfitting that is carried out under a system of operation within the exemption of section 22 of the PGD Act 2006 may be certified by the competent person identified in the system of operation who is responsible for th safety and compliance of the gasfitting carried out

(5)  Every person commits a Grad A offence who, being a person who is responsible for ensuring that gasfitting is certified under this regulation, fails to ensure that the gasfitting is certified in accordance with this regulation and regulations 46 to 48.

45  EXEMPTIONS FROM REQUIREMENT TO CERTIFY GASFITTING
Requirements to certify gasfitting do not apply to:
(a)  any work on any pipes or fittings suppliedwith liquefied petroleum gas from any portable gas storage container or contrainers that have, or togethr have, a maximum capacity of 15 kilograms of liequfied petroleum gas or
(b) any gasfitting that replaces a gas appliance with an equivalent gas appliance if the work on he replacement appliance, or the replacement appliance itself does not result in 1 or more of the following:
i)  repositionin gof pipework
ii)  repositioning of flue system components
iii)  a change in the installation pressure
iv)  a change in the gas type
v)  a significant change in the energy consumption
vi)  a change in ventilation
vii)  a change to the operation of the installation or;

(c)  any gasfitting that replaces a fitting in an installation with an equivalent fitting if the fitting is instrumentation and related controls and the work does not result in repositioning or disturbance of pipework, other than instrumentation and related controls pipework.

So basically a straight like for like with NO CHANGE TO even an inch of pipework or any of the i - vii may not need a certificate, but why wouldn't you want to do one?  If you've done the work, it's safe then we certify like for like as low risk and still do a cert and charge for it.   It also means you are covered for your work - hope this has been some help.  Interested in what others do?

Indeed it would be - thanks!  I have downloaded and printed and will chuck my old one away on Monday.  My first stab at the answer was correct, my second one not so much.   We certify everything regardless - and I subscribe to this theory - you can never over certify and best to be safe than sorry.