Previous attempts by police to get tips from the public have not resulted in any further leads.
"We simply do not know what has happened to Mr Ngata," said Detective Senior Sergeant Marcia Murray.
"We have no evidence at this stage that anything untoward has happened to him, however we are continuing to make enquiries."
Police said Mr Ngata may not want to be found, but encouraged him to get in touch and let someone know he was okay.
However Ms Murray said it was important to keep an open mind as to what might have happened.
"We'd like to hear from anyone who may know where Mr Ngata is, or has information that could help police."
Anyone with information should contact the Avondale Police on 09 302 6400.