School facilities were used to print copies of letters, although Mr Allen had advised that he was taking stress leave.
According to the charge laid against Mr Allen, the letter was critical of Mr Olver and used "inflammatory, offensive, and disrespectful" language.
It described the principal as someone who "discriminates, is prejudiced, segregating, and silencing" and implied the principal "was "bigoted and racist" and was prepared to sacrifice Maori interests.
The tribunal report said the letter also linked the principal "with apartheid", and accused him of prioritising self-interest to further his career and not being "committed to the best interests of Whanganui".
Mr Allen resigned from the school in February 2015 after working there since 2009. He now teaches at Ruapehu College.
The matter was investigated by the the Complaints Assessment Committee and the report says Mr Allen admitted the charge and accepted his behaviour amounted to misconduct.
The committee said that in view of Mr Allen taking responsibility for his misconduct, the only penalty required was censure. He was also required to pay some of the tribunal costs.
Whanganui High School staff and its board of trustees were not willing to comment on the decision yesterday and Mr Allen also did not wish to comment.