The 18-year-old shared his support for the ideology behind the Christchurch mass shootings and police made a decision to arrest the man amid concerns about his escalating behaviour.

Australian Federal Police Assistant Commissioner Scott Lee said the man held extreme right-wing views involving white supremacy and anti-Semitic beliefs.

He had used online platforms to share his views into the early hours of Wednesday morning ahead of his arrest.

He had allegedly spoken of a mass casualty event and his potential involvement, but the information wasn't specific.

He allegedly spoke of "general acts of violence".

The Joint Counter Terrorism Team began investigating the man in August amid concerning posts made on various social media platforms.

NSW Police Assistant Commissioner Mark Walton said the online comments urged acts of violence against "anyone who didn't look like him".

He said there is always a risk of escalation in such cases and police supported early disruption to avoid tragic outcomes.

"The NSW JCTT will continue to work together to ensure the community is protected from those who seek to do us harm, and those who think they can exploit the online environment to spread their extremist ideologies," he said.

"If we see activity that concerns us - as we have during the course of this investigation - we will act swiftly to ensure that activity does not escalate.

"I would like to take this opportunity to remind the community that they are our eyes and ears - so if you see something online, report it.

"Your information is often invaluable in assisting investigations such as this one."

Assistant Commissioner Lee said there were concerns such behaviour could result in a violent incident.

"The sole aim of today's activity was to ensure the safety of the community by preventing further planning, preparations or advocacy by this man that could have resulted in an attack in Australia," he said.

"The investigation into this matter remains ongoing, but we remain wary about the speed with which lone actors can progress from online activities to ones that impact the real world.

"It highlights the commitment and professionalism of the people and agencies involved in the JCTT, and their desire to protect the community by bringing people to account for their criminal actions."

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The man is expected to be charged with advocating terrorism and inciting others to commit acts of violence.

One nearby resident said a mum lived at the home with two male and one female children.

They said they kept their distance from the family.

The home has drug links.

Nearby residents expressed shock at the raids which involved a large number of marked and unmarked police cars.

Officers remain at the scene.

"The decision to arrest today was made as a result of an escalation that we saw in this male's online behaviour which continued up until this morning," Assistant Commissioner Lee said.

"That is why we made the decision we did to go overt to ensure that we could do all that we could to protect the public from any risk of violence, whether from this individual or from others in terms of these acts.

"He was only acting alone.

"This was only occurring in the online environment.

"A couple of days ago what we observed was an escalation in the tone which went to the support of a mass casualty event and potentially his involvement in that event.

"Then there was a post in the very early hours of this morning which actually expressed support for a previous mass casualty shooting that occurred internationally."

Police expected to charge the man with urging violence against members or groups and advocating terrorism.

NSW Police Assistant Commissioner Mark Walton said the online comments urged acts of violence against "anyone who didn't look like him".

He said people engaged in extremist behaviour online were often disengaged from society.

He said there is always a risk of escalation in such cases and police supported early disruption to avoid tragic outcomes.

"There's certainly a lot of research and a rich history, particularly in Europe and the US of young people moving towards these ideologies," he said.

"I think in simple terms it's around, they're looking for a cause, they're looking for an identity, looking for something to connect to.

"They're disengaged from the mainstream society.

"They're hero worshipping people that we would suggest in history should be forgotten."

Police say they will target extremist and violent ideologies, whatever those ideologies may be.

Assistant Commissioner Walton said investigators keep a close watch on what's happening online.

There are concerns about what COVID-19 has done to the online environment and the threat posed by extremists on social media sites.

Police didn't say which platforms an 18-year-old involved in Wednesday arrest used, but said he used multiple social media sites, closed groups, and readily available technology.

"In the environment we work in, we're not waiting for people to come to us obliviously," the assistant commissioner said.

"We have techniques that cause us to scan the digital environment, but we always rely on information from members of the community.

"There will always in hindsight be members of the community that will reflect that (they) should have had some concern about (a) persons's conduct and we will always urge people to come forward and assist us in the early disruption of people's mindset, the early disruption of people moving from thoughts to actions."

Assistant Commissioner Walton said it is often a "very complex, delicate decision" about when to move from monitoring the internet to making arrests.

"We will move earlier than later to ensure we get the best results and we avoid any tragic outcome," he said.

Wednesday's arrest was made as part of a joint terrorism taskforce involving federal and state police.

The Australian Security Intelligence Organisation and the NSW Crime Commission is also involved.

A federal police spokesman said there was no ongoing threat posed to anybody in relation to the East Albury case.