The NSW Joint Counter Terrorism Team announced two arrests on charges of financing terrorism on Tuesday. One was a 20-year-old male who was part of the original Operation Appleby disruptions in September 2014. At the time, these high-profile overt actions attracted some criticism for only resulting in two arrests; Tuesday's charges bring the total number of arrests to 14.

Appleby illustrates the delicate balance between disrupting terrorist plots to save lives - as occurred in 2014 - and making arrests on evidence of criminal action, which may only occur sometime later.

There's been a significant increase in terrorist plots against the West, including Australia, in the past two years. Since 2014 Islamic State (ISIS) has been tied to at least 75 plots against Western countries, with the United States the group's main target.

That’s the key finding of the report, Terror Gone Viral, released this month by the US House Committee on Homeland Security. In 2014 there were 19 ISIS-linked plots against the West. But last year the figure more than doubled to 48.

The high success rate - with more than 40 per cent of total plots executed - indicates the group will likely continue to initiate frequent attacks.

More than one-third of ISIS-linked terror plots were aimed at the United States or its interests overseas. These include the San Bernardino shooting in December last year, which killed 14 and injured many more, an attempted ISIS-inspired attack in Garland, Texas, outside a Prophet Muhammad cartoon contest, and numerous attempts to attack New York City landmarks.

ISIS-inspired attacks targeting the West aren't only growing in number. They're getting deadlier. In the second half of 2014, an average of one individual died in each successful attack. In the first half of last year the average death toll rose to 11.

The report notes that US counterterrorism agencies are increasingly concerned terrorists are "going dark": ISIS-linked attackers are encrypting communications or making data inaccessible to authorities. It found that the line between inspired and directed plots is getting blurrier: an individual can start off self-radicalised and later gain ties with ISIS.

The study found that ISIS is successfully "crowd-sourcing" its terrorism agenda by inspiring independent followers to conduct most of its attacks.

The majority of cases involve only a single suspect, some element of online radicalisation or engagement with ISIS's digital propaganda. But so-called lone-actor terrorists typically aren't acting alone: they're found to be engaged with ISIS overseas.

The study notes that terrorists overwhelmingly opt to use easily accessible weapons, such as knives or guns, over explosives. 

At least one-third of ISIS-linked plots against the West involved an individual who's gone abroad to train or fight in a terrorist safe haven. The report indicates this estimate is low as accomplice roles aren't always clear.

The US study confirms that Syrian refugee flows and fake passports are being exploited by ISIS to deploy terrorists to Western countries.

There are several key messages in this latest US study on the 75 ISIS-linked plots against the West over the last two years for Australia's counterterrorism strategy.

Australia is ranked equal-third with the United Kingdom as a target of ISIS, of those 22 countries attacked by ISIS outside the Syria-Iraq conflict zone.

We can continue to expect more attacks, underscoring the current terrorist threat level of "probable".

The list of countries targeted belies the argument that ISIS will leave you alone if you stay out of the coalition operations in Syria-Iraq: Spain, Indonesia, Kosovo and Switzerland all feature on the list.

The report confirms Australia is right to be concerned about foreign fighters playing a role in directing or participating in attacks back home. Our 110 known living foreign fighters remain a problem.

The trend towards a high success rate of executed plots underscores the critical importance of disrupting plots. The rapid turn-around time of those radicalising and willing to use violence, as well as the number of plots, are placing extreme demands on Australia's investigative capacity.

We need a counter-terrorism approach that provides effective powers and means to disrupt terrorist plots. 

Jacinta Carroll is head, counter-terrorism policy centre, Australian Strategic Policy Institute. Anthony Bergin is deputy director at ASPI.

Originally published on the Australian Financial Review.