Kia ora Geoff.
I would suggest contact the New Zealand Defence Force (NZDF) and obtain a copy of his service records or ask other members of your family for a copy in the event they have already done this. Basically because it takes some time for NZDF to respond and complete the request.
Secondly, I would suggest contacting the NZ Archives. There you can request a copy of the 28 Maori Battalion War Diary for the month of December 1943. It does cost money though for them to reproduce the diary in digital format for you. Alternatively, if you are near the Wellington region, you can go and visit the Archives and read the war diary yourself to see if your Grandfather is mentioned. I believe viewing records in person is free. The Official Histories were written based on the war diaries but they do not contain the full detail. Even if it does not name your Grandfather, it should give you a deeper insight into the battle than what the official history will.
Best of luck,
Brendon