Security agency ASIO would have to add at least 9000 staff – a six-fold expansion of its current size – at a cost of at least $1 billion a year to watch all potential terrorism suspects such as the Bourke Street killer round the clock, security experts and sources have said.

The bracing assessment was made as the Morrison government turned up its pressure on Muslim leaders to do more to detect and report extremism in their midst, provoking an angry response from Islamic community groups.

One national security source described as "impossible" the prospect of watching subjects in all of the estimated 400 priority terrorism cases that ASIO has.

The source said that ASIO instead uses a risk-management matrix, in which only a fraction of the people of concern to the agency are routinely physically surveilled. Others have their electronic communications monitored and have ASIO officers check on them with varying regularity to see whether they have changed their behaviour or stepped up their extremist activities.

That view was backed by Deakin University terrorism expert Greg Barton, who said it took a team of up to 10 officers to watch a suspect properly, including backroom support staff.