Two days after the Australian Bureau of Statistics began mailing same-sex marriage voting slips around the country, police were called to the University of Sydney when a small student-run stall was overwhelmed with protesters.

Sydney Morning Herald reports that a confrontation occurred around a stall believed to have been set up by the student-run Catholic society, where a small number of students were making speeches and holding signs reading “It’s OK to say ‘No’”.

An unnamed spokesperson for the Sydney University Catholic Society told the SMH that Vote No campaigners were "physically assaulted, verbally abused and repeatedly shouted down".

"We were told we cannot be on campus with such beliefs ... we maintain that university is a place where different opinions can and should be expressed without fear of safety," the spokesperson said.

According to an editor of student newspaper Honi Soit, Kishor Napier-Raman, “There were maybe 15 'Vote No' people there and hundreds of 'Vote Yes' people.”

Mr Napier-Raman told SMH this was the first protest against same-sex marriage he had seen on campus.

A spokeswoman for Sydney University was quoted saying "We understand that a number of people protested in favour and against same-sex marriage.

"The University of Sydney expects that any expression of views on this matter is conducted in a respectful manner."

Same-sex voting cards have begun arriving in letterboxes around the country this week. The ABS is encouraging Australians to return their survey forms by October 27. Forms will not be accepted after November 7.