Race Relations Commissioner Joris de Bres said Mr Portman was angry at seeing other cultures represented and wanted the school to echo his prejudiced views.
"He's basically gotten angry as he has driven by and without any further consideration he's sent them this email on the hope that they'll teach race hate.
"He's basically a bigoted passerby throwing a molotov email into the school."
Mr de Bres said the school was to be commended for putting up a range of flags showcasing the diversity of New Zealand society.
He praised its leadership for rejecting Mr Portman's views and called on the school to ignore the email.
"What he doesn't want is Maori. He's trying to couch this in national pride and a sense of belonging but what he's basically objecting to is the school having symbols of Maori visible."
"They should not consider it for a second."
The email attached to Mr Portman's complaint had been forwarded to the Human Rights Commission by complainants, Mr de Bres said.
Te Runanga o Ngai Tahu Otakou representative Tahu Potiki said he was "surprised that supposedly educated community and business leaders would get into such ignorant behaviour".
"The information is so erroneous, loaded and one-sided and many of those things can be disproven. I understand people can have attitudes worked up over time about the place of Maori in New Zealand society, but as as a community we have moved well beyond this sort of [rubbish]."
Mr Portman, a manager with Export Link Otago, told the Otago Daily Times he stood by his comments but regretted forwarding the email as it had the name of the person who had sent it to him.
That person had been the subject of abuse and received a written warning from his company, he said.
He admitted the forwarded email "was a bit inflammatory" in parts, including the lines:
"You rob us, convert our cars, rape our women and bash our elderly.
But if a white police officer shoots a Maori or a Maori gang member, or assaults a Maori criminal running from the law and posing a threat to society, you scream racism."
Mr Portman said he sent the email because the school principal was against National Standards but was "teaching all this Maori culture, which is basically culture that's been made up as it goes along".
He had sent the email in good faith and on behalf of his local community, and had hoped the flags would be taken down.
"We are all one people here and this division is not good for the country."
When asked if he was Maori, Mr Portman said he had connections with Waitaha but had chosen not to enrol with the tribe.
Asked if he had Maori blood, he replied: "I guess I have."
North East Valley Normal School board of trustees chairman Steve O'Connor said the school flew flags that represented the diversity within the school, and would continue to do so.
"We are a diverse community. We recognise that and we celebrate it and we want our kids growing up in the atmosphere of doing the same."
Mr Potiki said schools should be allowed to fly flags such as the tino rangatiratanga, as it was "an accurate reflection of where our community has evolved to. Kids are so much more aware of Maori culture, Maori language, the history of New Zealand. It is really very very good, and to have some guy drag us back to the dark ages ... it needs to be challenged."
