He said Mr Kaukau made an "obscene suggestion". Koloamatangi declined and was propositioned again.
Koloamatangi punched Mr Kaukau on the chin. Mr Kaukau fell backwards and banged his head on the pavement.
"Thus far, it could be dealt with by any list judge sitting anywhere in the country, however, in this case there were severe and unforseen circumstances."
Mr Kaukau has brain damage and spent weeks in hospital, two in an induced coma.
He also had operations to relieve pressure on his brain, including one where part of his skull was removed.
"It must have been extremely stressful and upsetting for the members of his family," Judge Wade said.
He said Mr Kaukau is now living in a rehabilitation centre.
"He has to learn things all over again, such as toileting and showering."
Judge Wade said the long-term prognosis for Mr Kaukau remained "uncertain".
He told Koloamatangi he would give him a shorter sentence for his early guilty plea and his clean record.
Judge Wade said Koloamatangi's prison sentence would not be more than one year and he would also consider home detention and community detention.
Koloamatangi's lawyer Jo Scott said her client was remorseful and "regrets his behaviour".
She said he would be willing to undergo a restorative justice programme, if the victim's family was willing.
Crown prosecutor Yvanca Clarisse said the family were "all at different stages of the grieving process".
She would contact victim services to see if they were able to take part.
Koloamatangi will be sentenced in December.
