Speaking to the BBC, Professor Richard Falk said he believed that up to now Israel had been successful in avoiding the criticism that it was due.
Professor Falk is scheduled to take up his post for the UN Human Rights Council later in the year.
But Israel wants his mandate changed to probe Palestinian actions as well.
Professor Falk said he drew the comparison between the treatment of Palestinians with the Nazi record of collective atrocity, because of what he described as the massive Israeli punishment directed at the entire population of Gaza.
He said he understood that it was a provocative thing to say, but at the time, last summer, he had wanted to shake the American public from its torpor.
"If this kind of situation had existed for instance in the manner in which China was dealing with Tibet or the Sudanese government was dealing with Darfur, I think there would be no reluctance to make that comparison," he said.
That reluctance was, he argued, based on the particular historical sensitivity of the Jewish people, and Israel's ability to avoid having their policies held up to international law and morality.
These and other comments from Professor Falk comments are, if anything, even harsher than the current UN investigator, John Dugard, who himself has been withering about Israel's actions.
A spokesman for the Israeli Foreign Ministry said that Israel wanted the UN investigator's mandate changed, so that he could look into human rights violations by the Palestinians as well as Israel.
If that were not to happen, the Israeli government may consider barring entry to the new UN investigator.
© 2008 BBC News