Malcolm Turnbull has criticised security arrangements in Europe ahead of the attacks in Brussels, arguing they had been allowed to "slip".

Key points: PM praises Australia's border security arrangements in wake of Brussels attacks

PM praises Australia's border security arrangements in wake of Brussels attacks Bill Shorten says it's "too soon" to speculate on the weakness of Europe's security

Bill Shorten says it's "too soon" to speculate on the weakness of Europe's security Australia's terrorism threat level remains unchanged

Australia's terrorism threat level remains unchanged Australians urged to reconsider Belgium travel plans

Opposition Leader Bill Shorten has responded to Mr Turnbull's analysis, saying it was too soon to "hand out advice".

The Prime Minister has praised Australia's border security arrangements in the wake of the attacks in Belgium, which have killed 34 people and injured more than 200.

But Mr Turnbull said he could not rule out the possibility of a future terrorist attack domestically.

"You cannot guarantee that there will be no terrorist incident," Mr Turnbull told ABC News.

"But I can assure Australians that our security system, our border protection, our domestic security arrangements, are much stronger than they are in Europe where regrettably they allowed security to slip."

Mr Turnbull directly related the events overnight with the wider approach to security in Europe.

"That weakness in European security is not unrelated to the problems they've been having in recent times," he said.

Mr Shorten cautioned against speculative comments about the factors behind the attacks.

"I think it's premature for the Prime Minister be telling the Belgians what they did wrong within 24 hours of what happened in Belgium," Mr Shorten said.

"For me, today, is about recognising that people have lost their lives — innocent people have lost their lives.

"No doubt the hard questions will be asked in coming days."

Australian threat level unchanged

The Federal Government is not increasing Australia's terrorism threat level, which remains unchanged at 'probable'.

The Community and Public Sector Union has postponed strike action at the nation's international airports that was scheduled to begin at midnight.

That decision followed pressure from the Prime Minister for the union to reconsider given the global security environment.

The Department of Foreign Affairs has updated its travel advice for Belgium, urging Australians to reconsider travelling to the country in the wake of the attacks.

There are no reports of any Australians being injured or killed.

Mr Turnbull said Australia and Belgium were united in the fight against terrorism, and he condemned the attacks as cowardly.

"We are allies in this battle," Mr Turnbull told the ABC.

"Indeed, Australia is allied with Belgium in this battle just as our forebears were 100 years ago in the fields of Flanders, in the First World War.

"We are absolutely shoulder to shoulder with Belgium."

Belgium 'under-resourced', European security expert says

Belgium does not have the right ratio of personnel to track persons of interest, Aberystwyth University senior lecturer in European Security Alistair Shepherd has told ABC NewsRadio.

"That's where I think the problem is coming from," Dr Shepherd said.

"The fact that they are under-resourced is shown by the fact that there was a network of people with quite advanced plans."

He said the problem was the number of people the Government was trying to track simultaneously.

"It's not necessarily that their quality isn't as good as a lot of European countries — they provided good intelligence in the end to Paris," Dr Shepherd said.

"It's the vast numbers of people they they're trying to follow.

"Belgium's one of the highest per capital senders of recruits into Syria for training and for fighting on behalf of Islamic State."