A legal scholar says accusations of copying against a national security judge underline the need for the authorities to offer more guidance on avoiding claims of bias.

Simon Young of the University of Hong Kong made the comments after High Court judge Wilson Chan was accused of making extensive use of the words submitted by one party in a case he ruled on. He earlier received a serious reprimand from the SAR's top judge over a separate case.

Speaking on RTHK's Hong Kong Today programme, Young said there's nothing wrong with accepting one side's arguments, but the judge must be seen to be fair.

"There's, in fact, two important duties that judges have. Of course, the primary duty is to act independently and impartially. But there's also a secondary duty, and you have to demonstrate that you've done that.

"If you have basically copied the verbatim – the full submissions – of one party, there may be real questions raised about your being able to demonstrate your independence and impartiality."

Young noted that issues of copying had arisen before in Hong Kong and elsewhere in the world.

He added that while copying part of a submission is common practice due to factors such as time and workload, judges should be seen to carefully consider all of the arguments presented before making decisions.

"One has to be more careful about how one prepares one's judgements in terms of demonstrating you've exercised an independent mind and acted impartially. Hopefully, we might get some further guidance from judiciary or the judicial institute on how we can better do that."

The Court of Appeal on Wednesday said it would reconsider a case involving two sons of the late co-founder of real estate firm Great Eagle Holdings, after finding that Chan incorporated extensively from one sibling's submissions in his ruling.

The Court of Appeal said it will make a judgement after considering relevant evidence on its own, and would give no weight or very little weight to Chan's judgement.

The national security judge had already received a serious reprimand from Chief Justice Andrew Cheung after the Court of Appeal ruled that he had copied the plaintiff's submission in his ruling on a trademark case. A retrial under a different judge was also ordered.