Australian Prime Minister Malcolm Turnbull on Wednesday said Europe had "allowed security to slip", as he questioned the EU’s Schengen passport-free zone in the wake of the Brussels attacks.

Australian Prime Minister Malcolm Turnbull on Wednesday said Europe had "allowed security to slip", as he questioned the EU's Schengen passport-free zone in the wake of the Brussels attacks.

Turnbull's comments came as Belgium's neighbors France, Germany and the Netherlands tightened border security after some 35 people were killed in Belgium's worst extremist assault.

The Australian leader said while it was up to Europe to set its policies, his nation's border protection measures and domestic security arrangements "are much stronger than they are in Europe where regrettably they allowed security to slip".

"That weakness in European security is not unrelated to the problems they've been having in recent times," he told the Australian Broadcasting Corporation, alluding to the wave of migrants who have flooded into the continent, many of them from Syria.

Turnbull added that the open-borders Schengen travel regime, which covers 26 of the 28 EU countries, meant "people are able to freely travel across borders within Europe -- that poses security challenges, coupled with clearly very porous external borders as we've seen plenty of evidence of that".

"My point really was to say that those arrangements have security consequences," he told reporters in Sydney.

The Schengen agreement, considered a symbol of European unity and prosperity, has been under scrutiny amid the unprecedented influx of migrants and revelations that some of the Takfiris in November's Paris attacks that killed 130 people came from Belgium.

Both the Brussels and Paris attacks were claimed by the Takfiri group, ISIL (so-called Islamic State in Iraq and Levant).

Canberra has been increasingly concerned about home-grown extremism and the flow of Australians travelling to the Middle East to fight with Takfiri organizations such as IS in Iraq and Syria.

Up to 49 Australians have been killed in the conflict in Iraq and Syria, with some 110 currently fighting or working with militant groups, Australia's domestic spy agency said in February.

Another 190 nationals were actively supporting ISIL back home through fundraising.

Australia has had three attacks since raising its terror threat level to high in September 2014.

An 18-year-old suspected militant was killed after he stabbed two police officers in Melbourne in September 2014, while a cafe siege in the heart of Sydney left a self-declared extremist cleric and two of his hostages dead in December 2015.

Last October, a 15-year-old boy was killed in an exchange of gunfire outside law enforcement headquarters in Sydney after he shot dead a civilian police employee.