Ellison (30), who made his first start for the Highlanders last weekend, is expected to rejoin Japanese club side Ricoh, where he spent two years before joining the Highlanders in early 2012.
Ellison, a centre, is in South Africa with the Highlanders and could not be contacted.
However, Highlanders general manager Roger Clark confirmed Ellison had a clause in his agreement which allows him to be released from his contract with the New Zealand Rugby Union at the end of the Super 15.
Ellison, who is married with two young children, has invoked that clause, Clark said.
Clark said it was disappointing to lose someone of the calibre of Ellison, who has played four tests.
''He was probably our best back last year and though he was out for a long time this season the way he played in his first game back last week showed what a quality player he is,'' he said.
Clark said he could not speak for Ellison but the money players were being offered by overseas clubs was just way above anything the NZRU could compete with.
''All of the guys that have gone so far this year, and there has been a lot, and we are talking about the best players in New Zealand, have been offered something like three or four times what we can.
''Unfortunately, we always knew with Tamati that he was never going to be here long-term with us. We just hoped it would be a little bit longer than this.''
Clark said overseas clubs and owners appeared to have bottomless wells of cash which they were prepared to use to get whatever players they wanted.
It was hard to blame the player, when they were being offered such big money to go and play for another side. Ellison's brother, Jacob, is playing in Japan after representing Otago last season.
Ellison appeared a certainty to make the All Blacks this season.
Clark said he was keen to keep backs Ma'a Nonu and Ben Smith for next year but the ''ball was in their court'', as they came off contract at the end of the season.