News that the UK has upgraded its terrorism threat level from 'substantial' to 'severe' will undoubtedly help the Abbott Government to prosecute its case for enhanced anti-terror legislation. However, Sam Roggeveen is right to point out that the Government has so far failed to adequately link the scale and nature of the problem with its suggested remedies.

While senior members of the Government, including Tony Abbott, Julie Bishop and George Brandis have all indicated that the terrorist threat posed by the conflicts in Syria and Iraq is a significant increase on anything Australia has previously faced, this is not the same as explaining how the new laws will address this threat.

Nevertheless, there is a strong case that most elements of the Government's anti-terror package (this is a link to a media release; the draft legislation has not been released yet) are both necessary and proportionate to the increased threat.

A caveat: I am focused here on the two most controversial legislative measures: (1) lowering the threshold for arrest without warrant for terrorism offences, and (2) the new offence of traveling to a designated conflict in which terrorist organisations are operating without a legitimate purpose.

I focus on these measures because, unlike other elements of the legislative package, they represent a specific and tailored response to the intelligence 'black hole' that Western security agencies are dealing with in Syria and Iraq.

This absence of information is due to a variety of factors including the increasingly sophisticated methods used by foreign fighters to disguise their travel to these theatres, their increased numbers as compared with previous conflicts, and the almost complete lack of Western presence of the ground.

The main problem is that the present legislation was conceived during a period in which the threat predominantly emanated from 'home-grown' terrorism as demonstrated in cases like Operation Pendennis and the plan to attack Holsworthy Army Barracks. Whereas the terrorism offences introduced by the Howard Government have proved adequate in accounting for such plots (with 38 people prosecuted in Australia as a result of CT operations and 22 people convicted under the Criminal Code Act 1995), they have proven far less effective against Australians based overseas.

For example, the Australian Federal Police faced substantial challenges in preparing the prosecution of Jack Thomas upon his return from training with al Qaeda in Afghanistan. Indeed, the limitations associated with conducting the investigation in Pakistan (where Thomas was detained) ultimately led to the overturning of his conviction for receiving funds from al Qaeda. It is reasonable to assume that there are other cases of returning extremists that have never seen the light of day due to the existing legislative constraints.

When the inadequacies of the current regime are matched with the fact that the Syria and Iraq theatres contain more Australians than all previous extremist conflicts combined, the rationale for the new laws becomes clearer.

While the threat level in Australia hasn't changed (at least for the time being), the nature of the threat has. The significant number of Australians fighting for Islamic State (and let's not forget Jabhat al Nusra as well) creates the opportunity for these individuals to link up with their countrymen and potentially plan an attack on Australia from offshore. Such a scenario would make it infinitely more difficult for Australia's security agencies to thwart an attack.

Moreover, under the current set of laws, it is conceivable that the AFP would be unable to arrest a suspect upon reentry to Australia, even where that individual was known to have fought with a proscribed terrorist organisation. This is because the evidentiary requirements for the existing terrorism offences are rightly high, while the ability of law enforcement agencies to collect evidence in countries such as Iraq and Syria is limited. True, there are arrest warrants out for the high profile IS fighters Khaled Sharrouf and Mohamed Elomar, but this is almost certainly on the basis of their social media use rather than from evidence gathered in Syria. There are many more Australian extremists in the Levant who stay away from social media but maintain the same abhorrent world view as these two. The lack of warrants for these other extremists is instructive.

In short, the nature of the threat has evolved in the last decade. The solution is for tougher measures that fill the gap between the law and the new paradigm.