Authorised woodburners have been designed and tested to ensure that they meet emissions and efficiency limits set out in the National Environmental Standards for Air Quality. Modifications may result in more emissions being discharged or a decrease in efficiency, which will invalidate the authorisation.
Modifying your woodburner without the advice of the manufacturer may also create a fire hazard or other safety risks.
See more information at https://environment.govt.nz/acts-and-regulations/regulations/national-environmental-standards-for-air-quality/