The Tonga Law Society says the parliament should not have passed a constitutional reform without referring it to public consultation.
The legislature unanimously passed Clause 89 to the Constitution, which will require judges to consider Tongan traditions, custom and culture when making their court decisions.
The acting Justice Minister, Samiu Vaipulu, said the matter was not put to the public because no law was actually being changed.
But the President of the Law Society, Sione Fonua, said it should have been vetted by the public, because the issue was very complex.
"The implication of something that's used to somebody's advantage, using the culture, it's a very serious issue. So that's why we were concerned and it should have been discussed publicly and allowed submissions," he said.
Fonua said there was already talk about how prospective MPs could treat voters to items like kava, defending it as an expression of Tongan culture.
"They have already talked about allowing the candidate to donate some sort of kava and also other things that they consider is a part of our culture, when you are going campaigning.
"So it is not an easy issue. Are you allowed to do that? Does it mean you are buying the vote?"