If Don Brash wanted to speak on an American campus we'd be hearing a different story.

That is according to US constitutional law expert Kent Greenfield, who says public institutions in that country have an obligation to provide a platform for speakers.

He told Mike Hosking private universities can be as discerning as they like but public ones have to do everything in their power to allow speeches to go ahead.

"Here at least the first amendment..requires not just the allowance of speakers but the protection of those speakers."

Greenfield says speakers at public institutions in America can expect certain rights.

"The city or the public university has an obligation to provide security to make sure that the people speaking are free of danger."

He says speakers who threaten others are not included.

"Threats and harassment are not protected speech, so if a speaker is threatening specific individuals or harassing individuals then that is not protected speech, but if what you are saying is what is considered offensive then we, on the whole, protect content."

LISTEN ABOVE AS KENT GREENFIELD SPEAKS TO MIKE HOSKING