An Asian hit man with a low IQ did not get a fair trial six years ago due to his intellectual impairment and lack of ability to participate in his own defence properly, his lawyer says.

Wan Yee Chow, 60, had been jailed for life with a 17 year non-parole period in the High Court in Auckland in 2007 for the murder of triad enforcer Tam Yam Ah by shooting him once in the chest.

Chow, known as Tall Man which is the translation of his Cantonese nickname Gao Lo, was to receive $10,000 for being the hitman. He is appealing his conviction and sentence. The Court of Appeal reserved its decision.

His lawyer Michele Wilkinson-Smith said it was clear the extent of his impairment was not known at the time and he could not participate in his own trial.

She said his bill of rights had been breached as he did not get a fair trial and he should now have a retrial ordered.

Wilkinson-Smith said Chow was only able to give rudimentary instructions to his lawyer and he was given no assistance to help him understand what was going on.

The Court of Appeal heard from Dr Jeremy Skipworth who said testing showed Chow's IQ was perhaps near 70 which was regarded as the cut off for intellectual disability.

He said Chow scored just above that and that his biggest problem was his information processing speed.

Wilkinson-Smith said his disability was not put before the jury or his inability to read English. However Dr Skipworth did think Chow had understood the case against him but might have struggled to follow the trial.

He said Chow had experience with other court cases which would have helped him.

An interpreter who assisted at Chow's trial said she was not made aware of any disability.

Chow came to New Zealand from China in 2003 after being imprisoned in re-education camps and has a post traumatic stress disorder. He has previous convictions for committing a dangerous act with fire with intent to injure and wounding.

Wilkinson-Smith said Chow always wanted to appeal but was held up by language difficulties. He had only received communication from his then lawyer in English.

She said he was not assessed as to his fitness to stand trial before the case was heard.

Crown lawyer Cameron Mander said the language difficulties were well known at the time. His lawyer was aware of it at the time and satisfied he got complete instructions for trial.