Govt, Labor as one on Sydney terror attack

The siege in Sydney's Martin Place has won the government support from the White House and the federal opposition.

Monday's hostage situation has already attracted international attention, with US President Barack Obama briefed.

A White House official said Lisa Monaco, the president's top counter-terrorism adviser, had spoken to him about events in Sydney.

Prime minister Tony Abbott urged Australians to go about their business.

"The whole point of politically-motivated violence is to scare people out of being themselves," he told reporters in Canberra.

"Australia is a peaceful, open, and generous society. Nothing should ever change that.

"And that's why I would urge all Australians today to go about their business as usual."

Mr Abbott said he could understand the concerns and anxieties of the Australian people but thoughts and prayers must above all go to those caught up in the siege.

Mr Abbott said he had chaired a meeting of the National Security Committee of cabinet briefed by NSW Premier Mike Baird and NSW police commissioner Andrew Scipione.

"I think I can also commend the people of Sydney for the calmness with which they have reacted to this disturbing incident," he said.

Mr Abbott said he had received support from a number of international leaders.

Canadian Prime Minister Stephen Harper tweeted his country's "thoughts and prayers are with our Australian friends", two months after Canada was rocked by a lone wolf terrorist in Ottawa.

Indian Prime Minister Narendra Modi also took to twitter, saying "such acts are inhuman & deeply unfortunate. I pray for everyone's safety".

New Zealand Prime Minister John Key contacted Mr Abbott with a message of support.

"I've contacted Prime Minister @TonyAbbottMHR to let him know our thoughts are with the people of Australia today," Key tweeted.

Opposition leader Bill Shorten said he spoke to Mr Abbott and Australians could be assured the opposition and government were as one when it came to maintaining Australia's safety and security.

"Australians are shocked but we won't be shaken," the opposition leader told reporters in Melbourne.

"We face moments like this with resolute spirit. At times like this, it's more critical than ever that the Australian community be solid, stick together, be united."

Acting Greens Leader Adam Bandt said their thoughts were with all those held hostage, their families and friends.