The head of Australia's domestic intelligence agency has confirmed investigations into hundreds of young Australian-Lebanese men who have joined the fighting in Syria.

ASIO director general David Irvine says the fear is the young men are becoming radicalised by their experiences and that they may present a future terrorism risk once they return to Australia.

"We continue to be concerned about young Australians going overseas to fight on battlefields that don't necessarily have a lot to do with Australia," he said.

"We are also concerned that young Australians go overseas and become quite severely radicalised in the extremist Al Qaeda-type doctrines.

"Our first and foremost concern is for the safety of those young Australians.

"But secondly, people who do develop the sort of skills that terrorists can use and more importantly the commitment that drives terrorism, that remains a concern."

Mr Irvine says most of the Australians are young males going to Syria either to fight or support the fighting in another way.

He says most of them were born in Australia or came to Australia at a young age.

The ABC understands the majority of those young fighters come from families with strong ties in the north of Lebanon, particularly from the area around Tripoli.

Many of them have family business interests in the regions and also family connections that reach across the border into Syria.

The potential for further radicalisation has worried ASIO for some time, but the number of those that are being directly investigated has doubled in the past year.

The war in Syria shows no sign of ending and, as one source said, these boys all have Australian passports and they can come and go as often as they like.

But while the number of Australians heading to Syria is in the hundreds, Mr Irvine says this does not mean all of them are directly involved in fighting.

"That does not mean that hundreds of people go overseas, join the Al Qaeda affiliates, get an AK-47 and go and fight, many of the people are providing supporting functions, health, medical or whatever," he said.

"They're supporting the struggle in ways that don't always involve picking up a gun."

ASIO made the comments on Monday after conducting the first - and more than likely last - tour of the new controversial ASIO building on the shores of Lake Burley Griffin in Canberra.

Attorney-General Mark Dreyfus was also there and echoed Mr Irvine's concerns.

"Some of these Australians regrettably are going to assist or work with or join terrorist organisations that are proscribed in Australia," Mr Dreyfus said.

"That's against Australian law for good reason and that's obviously a concern."

Mammoth building

The new ASIO building sits on the shores of Lake Burley Griffin in Canberra. ( ABC News )

The new ASIO building has a mammoth presence, sitting across the water from Parliament House - a testament to its importance in the overall hierarchy of government infrastructure.

It is what many would expect: a high-tech security building with state-of-the-art biosecurity screening booths at the staff entrance.

At its heart is a 24-hour operations centre that can run off the grid for up to three days at a time.

It has its own water supply, sewerage system and generators fuelled by a 600,000-litre underground diesel tank.

For many it has become something of a metaphor for the unchecked growth of the intelligence community since the September 11, 2001 terrorist attacks in the United States.

Built at a staggering cost of more than $600 million, the ASIO building was never scrutinised by the parliament - as other big public projects usually are - and it was also exempt from the usual public consultation process.

But Mr Irvine says the cost is justified.

"We are still conducting as many investigations into possible terrorism in Australia today as we were four years ago or five years ago," he said.

"The issue has mutated over the years... but as you know there are still extensive Al Qaeda mutations throughout the world.

"From our reading of what is happening in Australia, we have every reason not to be hugely alarmed or paranoid, but to be alert to the fact that there continue to be people within our midst who would plan ill for ordinary Australians."

The intelligence chiefs say the building is designed to meet Australia's intelligence needs for the next 60 years, in an environment that is as dangerous as ever.