Staff in the Department of Foreign Affairs and at the Australian High Commission have been working frantically with the families of the two Australians and with British officials.

Prime Minister Malcolm Turnbull confirmed on Monday that in addition to Brisbane woman Candice Hedge and Darwin man Andrew Morrison, who were both injured in the stabbing phase of the attack, two more Australians may have been caught up in the incident.

But Mr Turnbull was tight-lipped about the condition of the second pair of Australians, saying only that authorities were "in close touch" with their families and that it would not be appropriate to say more.

There are fears for 21-year-old Brisbane woman Sara Zelenak. Photo: Facebook

One of the two unnamed Australians may be Sara Zelenak, 21, who was last seen on London bridge and has not answered her phone since, despite calls and texts from worried friends and family.

A Facebook post written by a family friend titled "Missing from London Terror Attack" has been shared more than 500 times.

The Prime Minister also backed his British counterpart, Theresa May, who has called for greater global regulation of cyberspace to prevent social media platforms being "safe spaces" for propaganda.

He pushed back against his predecessor, Tony Abbott, who has called for police to be given greater "shoot-to-kill" powers to deal with terrorist incidents and that the military should take the lead in combating complex terrorist incidents.

Mr Turnbull argued the "view being put around that police do not have an ability to shoot-to-kill . . . is quite untrue".

"In terms of the role of the Defence Force in terrorist incidents and terrorist attacks, that is a matter that is being reviewed," he said. "There are procedures, of course, that would enable the SAS, for example, to be brought in to a siege situation."

On the two as-yet unidentified Australians, Mr Turnbull said the government had "very real concerns. But at this stage, we're not able to say anything more".

"We have been in touch with their families, in close touch with their families, as we seek to find confirmation of the circumstances of the two other Australians."

Candice Hedge, injured in the London Bridge and Borough Market area attacks, has tried to reassure her friends and family that she's going to be fine. Photo: Facebook

Ms Hedge, 34, one of the two Australians who has been identified, has been left unable to speak after being stabbed in the throat.

"She can't talk. She's been stabbed around her neck, her throat. She's all bandaged up." her mother, Kim Del Toro, told Fairfax Media. "She went into surgery, but she's going to be fine, thank goodness."

Darwin man Andrew Morrison, who was caught up in the terror attacks.

Mr Morrison's sister, Katrina, said he was safe and a flight home was being arranged.

On Prime MInister May's call for greater regulation of social media, Mr Turnbull said she had "put her finger on a very important point, that there is too much tolerance of extremist material on social media and that, ultimately, requires co-operation from the big social media platforms, in particular, Facebook and Twitter".

He suggested social media messaging companies should assist authorities by providing access to encrypted communications platforms such as WhatsApp and Apple iMessage, which are used by billions of people.

At least seven people were killed and 48 injured in the weekend incident, which targeted the area surrounding Borough Market and London Bridge in the centre of the city. Three suspected terrorists were killed and a dozen arrests have been made.

Speaking after the terror attacks in London, Mr Abbott also said the West had to "avoid any spirit of surrender" and that "Islamophobia hasn't killed anyone. Islamist terrorism has now killed tens of thousands of people".

In fact, last month two men were killed in Portland, Oregon after they intervened to try to stop a passenger on a light-rail train from yelling anti-Muslim hate speech.

Mr Turnbull, who met with national security advisers and cabinet colleagues on Sunday after the attack, said the most important tool to counter terrorism was Australia's intelligence agencies.

"That is why it is very important for our intelligence services, ASIO - working with the Federal Police and, of course, the state and territory polices - to be able to be alerted to these plots, as they develop, so that they can be uncovered," he said.???

With Steve Lillebuen, Rachel Browne, Lynette Eyb

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