The Board of Inland Revenue functions through powers given to it by Parliament. These are defined by statute and regulated within the rules of the common law. Legislation and courts set limits on the board. There are things it must always do, things it cannot do and things that allow its discretions to be reviewed and moderated.
Taxpayers can challenge assessments to the tax tribunal; also, in certain circumstances, they can sue direct in the High Court to make recoveries and/or impeach board practice. This is a constitutional arrangement and changing it would require the full constitutional majority. An alternative system may not meet basic legal standards and if such a thing was ever made “constitutional” it could itself amount to an illegality notwithstanding its own “constitutional enshrinement”.
The proposed Revenue Authority will afford the tax assessor to negotiate levy with discursive freedom and spectral remit beyond purview of law. This has serious implications, including where influential businesses and groups win concessions and boundaries that are denied to others; and other things of such stripes that essentially can not have legal footing and remain nebulous.