After seeing Christchurch shooting on TV, the alleged gunman's parents approached police.

The family of an Australian man charged over the Christchurch mosque massacre approached NSW Police after viewing footage of the terror attack.

At least 49 people died in the attack on two mosques on New Zealand’s South Island on Friday and about 40 people are in hospital.

The gunman posted a 74-page “manifesto” online before the attack, and a 17-minute video was also uploaded of him arming himself and entering a mosque where he started shooting.

“My understanding is the family (of this man) did approach NSW Police after the incident was on TV,” NSW Police Commissioner Mick Fuller said in Sydney on Saturday.

Three people have been arrested including 28-year-old personal trainer Brenton Harrison Tarrant, from Grafton in NSW, who has been charged with murder and remanded in custody until April 5.

Mr Fuller has asked people not to watch Tarrant’s “disturbing” vision of the massacre circulating online.

He also urged Australians to tell law enforcement agencies if they see anything worrying posted on the internet, even if it’s related to their own son or daughter.

“If there’s information that’s concerning and you think it may just be a mental health issue, it could be a serious criminal issue and we would ask that you come forward,” he said.

NSW Police are conducting a “two-pronged investigation” which involves assisting New Zealand agencies and making sure the Australian terrorist “hasn’t slipped through the cracks”, Mr Fuller said.

He said there were “no active threats” in NSW linked to the shooting, however the national terrorism threat level remains at “probable”.

The commissioner joined other senior police at an 80-strong meeting with religious leaders, including Grand Mufti Dr Ibrahim Abu Mohammed, at the Australian National Imams Council.

Police patrols and visits to mosques around Sydney’s metropolitan area have been increased and will continue in the coming days and potentially weeks.

“There is no threat to any mosques, but we know that seeing police makes people feel safer,” Mr Fuller said.

ANIC spokesman Bilal Rauf said the “positive engagement” recognised an incident that has created a “heightened sense of concern” amongst Australian Muslims.

“The terrorists that do this do it for one objective. That is to divide, that is to fragment our society and that is precisely what we need to guard against,” he said.

“We need to be calm, we need to be united in our language, in our actions and our conduct.”

Mr Rauf hopes people are not deterred from National Mosque Open Day on Sunday, as avoiding such celebrations would mean “in a sense, the terrorist’s won”.

NSW Premier Gladys Berejiklian visited Lakemba Mosque on Friday night for a vigil with the Muslim community.

“I want to stress that an attack on one section of our community is an attack on all of us and we will band together, and we will heal together,” she told reporters on Saturday.

The premier said the closeness of the two countries, with shared values of freedom, diversity and democracy, makes it feel “as though it’s occurred on our own soil”.

“I know that words cannot describe the pain and anguish of the family and friends of the victims but our heart goes out to each and every one of them,” Ms Berejiklian said.