Haw. Rev. Stat. § 605
The judge presiding at the trial may not testify in that trial as a witness. No objection need be made in order to preserve the point.
HRS § 605
RULE 605 COMMENTARY
This rule is identical with Fed. R. Evid. 605, the Advisory Committee's Note to which says:
The solution here presented is a broad rule of incompetency, rather than such alternatives as incompetency only as to material matters, leaving the matter to the discretion of the judge, or recognizing no incompetency. The choice is the result of inability to evolve satisfactory answers to questions which arise when the judge abandons the bench for the witness stand. Who rules on objections? Who compels him to answer? Can he rule impartially on the weight and admissibility of his own testimony? Can he be impeached or cross-examined effectively? Can he, in a jury trial, avoid conferring his seal of approval on one side in the eyes of the jury? Can he, in a bench trial, avoid an involvement destructive of impartiality? The rule of general incompetency has substantial support.