Turnbull spoke from Turkey where the G20 is due to begin, at the same time the French flag was flown on top of Sydney's Harbour Bridge as a symbol of Australian support for the French people.
The famous blue, white and red flag was hoisted alongside the Australian flag in a show of solidarity that followed the Opera House emblazoned with French colours on Saturday night.
Speaking from Antalya, Turnbull said many countries had a common cause to defend themselves against extremist attacks.
"The attack in Paris was an attack on all humanity. We are in a common cause and we have a single common purpose, to defend our values and our way of life - and to defeat these terrorists and their assault upon us," he said.
Turnbull said bringing a unified approach to defeating Islamic State, which has claimed responsibility for the Paris attacks that killed 129 people, would be top of the list of items at the G20.
"A collective response of co-operation, of mutual support with a common purpose of combating and defeating terrorism, of this kind, has to be undertaken and it will be one of the major items on the agenda of this G20," he said.
He called for a political solution besides military action to solve the crisis in Syria, where France, the United States, Australia and a coalition of countries have conducted air strikes against Islamic State.
"It is very important that Daesh is confronted and defeated militarily, but longer term, the stability of Syria, the stability that will enable those millions of refugees to return to their homes, will depend on a political solution - and that of course is what is being pursued in the talks in Vienna, and elsewhere," he said.
Daesh is an Arab acronym for Islamic State.
Turnbull left open the prospect of a greater Australian commitment in the war against Islamic State in Syria and Iraq, and said Australia could also consider involvement in an international peace-keeping force if the situation in Syria could be stabilised.
He said Australians stood shoulder to shoulder with the French people.
In Hobart, the family of a Tasmanian teenager shot in the Paris terror attacks said she was scared and overwhelmed after undergoing surgery in hospital.
Emma Parkinson, 19, sustained a number of gunshot wounds to her hip at the Bataclan theatre.
Parkinson's aunty, Sam Gunner, told reporters she was due to be reunited shortly with her mother.
"Emma was in attendance at the Bataclan theatre. We are told Emma sustained a number of gunshot wounds to her hip. Emma was taken to the local hospital for treatment where her mum was able to speak to her very briefly on her mobile phone. As can be expected, Emma was scared and she was very overwhelmed, and she was in desperate need of a hug from her mum," she said.
Gunner said her family spoke with Parkinson briefly after she arrived at hospital and knew she was alive.
"Oh it's absolutely devastating, for everybody involved, obviously. As I say, our thoughts and out condolences are to the family and friends of the victims that were killed in this horrific event. Emma will be devastated for them and obviously also the support workers, the people in the hospitals, the people working in the ambulances, it has just such an enormous widespread effect and it's absolutely devastating for everybody involved."
Parkinson, who speaks fluent French, had moved back to Paris from Germany ten days ago.
