There was no direct comparison to the Nazi Holocaust, in which six million Jews were killed.
But Jewish Council president Stephen Goodman said calling what happened a holocaust was ignorant and improper.
"It tries to elevate Maori grievances by associating with the Holocaust, and I find it very hard to draw a comparison between the European colonisation of New Zealand and plain genocide," he said.
The genocide in Rwanda could be a holocaust, but the New Zealand context was different.
"It works on trivialising the Holocaust and diminishing the suffering and sheer horror of it all. [Maori] have every right to draw attention to their issue, but there are ways to go about it, and there are inappropriate ways - this is a highly inappropriate way."
New Zealand children did not learn enough about the Holocaust's history at school, Mr Goodman said - echoing Mr Opai's call for more education about Maori suffering.
The comments have been highlighted in Britain's Daily Mail under the headline: "Academic sparks outcry for comparing Britain's colonisation of New Zealand to Holocaust."
Disability Issues Minister and Maori Party co-leader Tariana Turia drew similar criticism in 2000 for mentioning "the holocaust suffered by indigenous people, including Maori, as a result of colonial contact and behaviour".
DEATH TOLL
NZ colonisation
* The Maori population fell by 40 per cent.
* More than 21,000 killed in warfare and 120,000 other deaths.
* Causes: War, disease and deprivation of resources.
Nazi Holocaust
* The European Jewish population fell by 60 per cent.
* Six million people, mainly Jewish, were murdered in death camps.
* Cause: Mass exterminations under the Nazi Party.
Sources: Te Ara, Telegraph