Newspaper columnist Andrew Bolt was boiling with anger when he entered the witness box on the second day of a racial vilification case, stung by comparisons of his articles with Hitler's Nazi Germany.

The controversial News Ltd commentator is being sued under the Racial Discrimination Act by a group of Aborigines over a series of articles he wrote in 2009.

Yesterday the court heard the articles questioned the motives of light or white-skinned people who identified themselves as Aboriginal, implying they did so for personal gain.

Bolt listened on Monday as Ron Merkel, QC, the counsel for the nine Aborigines bringing the class action, said his articles on racial identity took a eugenics approach and echoed the Nuremberg laws of 1935.

While Mr Bolt had been warned by Justice Mordecai Bromberg not to use his testimony as a public forum, the journalist vented his anger before his cross-examination today.

He said any statement linking him to the Nuremberg laws and the Holocaust "were false and grossly offensive".

"Mr Merkel crossed the line," Bolt said, adding that he was a vigorous opponent of the eugenics movement.

Bolt also rejected the suggestion a series of articles he wrote about light-skinned Indigenous people were inaccurate.

He told the court race was a "hazy concept" and it would be a mistake to define it for all time in a court decision.

He also rejected the suggestion he was selective in researching his subjects' backgrounds, telling the court that reading between the lines as a journalist is a good thing "when you come to the right conclusion".

When asked if he was aware that his articles had caused distress, Bolt replied: "In terms of distressing people when you disagree with them, I know all about that. That's why I'm here."

The journalist told the court he did not contact any of the subjects of his articles before publication and considered them a response to comments they had already made on the public record.

An earlier witness, Professor Larissa Behrendt, said Bolt had used a photograph of her in an article picturing her with dyed blonde hair and commenting on her German heritage.

Professor Behrendt, 43, said that while her grandfather was born in England she had no knowledge of German ancestors, although she admitted her surname was German.

She described herself as an Aborigine and said her father was an Aborigine and her mother was a white Australian.

She told the court that she knew of a three-point test to decide if someone was an Aborigine in order to claim benefits.

It covered a person's Aboriginal descent, their acceptance among the Aboriginal community and their own self-identification of being an Aborigine.

She admitted it would be ludicrous to say you were Aborigine if you had to go back seven generations to find black heritage.

The counsel representing the applicants is seeking a public apology from Bolt and a ban on republishing the articles, which appeared in 2009 and 2010.

The applicants say they were offended, insulted, humiliated or intimidated by Bolt's articles.

The trial continues in the Federal Court in Melbourne.

- ABC/AAP