The story was planned for next Monday and was a return to Native Affairs' most controversial investigation subject which probed management and spending at the trust.
The investigation won the Best Investigative Reporting award at the 2014 World Indigenous Journalism Awards with judges saying it was "an excellent example of the rigour with which we need to conduct ourselves as journalists."
The show has also raised concerns among some in Maoridom's old guard because of the energetic questioning of an establishment organisation led by matriarch Dame Iritana Tawhiwhirangi.
The story was under scrutiny by the Broadcasting Standards Authority at the time of Mr Maxwell's own controversial appointment and sparked fears he had been hired to control a newsroom which was making powerful figures uncomfortable.
Mr Maxwell promised then he would not interfere with reporting on the channel. "If indeed there are reins on Native Affairs or Te Kaea, I'd like to unleash them."
The Herald has been told he has since raised concerns about ratings for Native Affairs. He told the Herald last year "any criticism from me (about the kohanga reo story) it is about the tone" of approach to establishment figures.
A July 2014 report for the channel's executive team, commissioned after Mr Maxwell's arrival, raised concerns around the style of reporting.
The Grafton Group report, released to the Herald under the Official Information Act, said "areas of concern" included "tensions" over the "Maori view of how to report" and "what reporting is from a Tikanga perspective".
Explaining the tikanga reporting concept, the report said it should "not challenge and critique one another".
In the wake of Mr Maxwell's arrival, two of Maori Television's most highly regarded staff - Carol Hirschfield and Julian Wilcox - left the broadcaster.
Mr Wilcox last night praised Forbes' contribution to the show. "It's a huge loss for Maori Television that someone of Mihi's credibility, journalistic experience and expertise is lost to the organisation. It's not a good time for news and current affairs in New Zealand television."
Mr Maxwell would not be interviewed yesterday. Maori Television issued a statement by news and current affairs boss Maramena Roderick saying Forbes made a "significant contribution to the kaupapa of Maori Television".
Forbes was praised in the latest Metro magazine as "almost single-handedly for setting the Maori journalism agenda".
In a piece celebrating the most influential Aucklanders, Morgan Godfrey wrote: "Forbes is unafraid to hold the powerful figures in Maori society to account: just as the Kohanga Reo National Trust.
"Yet her influence also comes the collective behind her. It's not her office that enhances her mana so much as the quality of her colleagues."
