The president of Australia's New South Wales Anti-Decimation Board said on Monday that racial cohesion is facing it's greatest threat in 30 years as violence marked anti-Islam and countering anti-racism protests at the weekend.

Hundreds of anti-Islam and anti-racism protestors rallied against each other at Reclaim Australia protests across the country at the weekend, with five people arrested in Sydney on Sunday, a day after violence erupted on the streets of Melbourne.

President of the NSW Anti-Discrimination Board, Stepan Kerkyasharian, said something similar to the extreme 2005 race riots in the Sydney suburb of Cronulla was "not necessarily off the plan" after watching the events at the weekend.

Widely condemned by Australia's political leaders, the Cronulla riots were a series of sectarian clashes in December 2005 where racial tensions boiled over after a series of escalating social incidents.

Australia's race discrimination commissioner Tim Soutphommasane backed Kerkyasharian's fear on Monday, calling for people to remain calm and be respectful of each other.

"It is a source of concern that we're seeing organized extremist elements operating in public ... we need to avoid such groups being emboldened and taking matters to the next level," Soutphommasane told local radio.

Two people were injured when more than 150 anti-Islam protestors marched in Martin Place in Sydney on Sunday, some with the Australian flag draped over their shoulders and others in ancient Greek army costume.