A judge, called a "hater" by Donald Trump for his handling of a lawsuit related to the businessman's Trump University real estate school, has unsealed documents related to the case.

Trump, the presumptive Republican presidential nominee, is fighting a lawsuit that accuses his school venture of misleading thousands of people who paid up to $35,000 for seminars to learn about the billionaire's real estate investment strategies.

In an order signed on Friday, US district judge Gonzalo Curiel said that materials, including Trump University procedures on dealing with students and the media, should be unsealed.

He noted that they had already been published by the media organization Politico and that a magistrate judge described them previously as "routine" and "commonplace."

At a rally in San Diego on Friday, Trump criticized Curiel for his handling of the Trump University case.

"I have a judge who is a hater of Donald Trump. A hater. He's a hater," Trump said.

"We're in front of a very hostile judge. The judge was appointed by Barack Obama," Trump said, adding he believed Curiel was Mexican.

Curiel is an American who was born in East Chicago, Indiana, and graduated from the Indiana University School of Law.

Trump has drawn criticism for his comments about immigrants from Mexico, some of whom he has said were criminals and rapists.

He has proposed building a wall along the US border with Mexico to prevent illegal immigration and requiring Mexico to pay for it.

Hispanics are a critical voting bloc in US presidential elections.

Last week, Trump knocked one of the highest profile Hispanic women in the Republican Party, criticizing New Mexico Governor Susana Martinez for her handling of the state's economy.

Her office said his criticisms were not substantive. Martinez has been touted as a potential vice presidential pick for a Republican ticket.