Responding to a deadly attack in Barcelona that has claimed at least 16 lives, Mr Turnbull said the love and prayers of Australians were with the victims and their families.
This is a global battle against terrorism," Mr Turnbull told reporters in Canberra on Friday.
Foreign Minister Julie Bishop said earlier that 16 people were killed in the attack, which took place on Thursday when a van careened into pedestrians in the Spanish city's tourist hub of Las Ramblas.
Scores of people were injured including three Australians, with one of those a woman hospitalised in a serious but stable condition.
Mr Turnbull said plans for keeping Australians safe domestically, including around public spaces, were constantly being adjusted.
"I can assure you - we are relentless."
Australian Security Intelligence Organisation (ASIO) director-general Duncan Lewis, appearing alongside the Prime Minister, said intelligence-gathering was essential to prevent attacks in Australia.
Working with the Council of Australian Governments would also be important in the future, Mr Turnbull said.
"We will look at the full range of counter-terrorism measures," he told reporters.
ASIO deputy director general, Tony Sheehan responded to a question about the vulnerability of Australian airports, particularly following the recent counter-terrorism raids across Sydney.
"Those matters are always under consideration by our agencies, Office of Transport Security and other relevant agencies," he said.
Shadow foreign affairs spokeswoman, Penny Wong, was pressed for comment and said it was an "unfolding" situation.
"I'm going to leave it to the government to respond to details, because obviously they have information to hand as it comes in," Ms Wong said.
New counter-terrorism measures were introduced in Melbourne in June, including road bollards along Bourke Street Mall and the northern edge of Federation Square.
A siren system was also introduced as part of the $10 million safety upgrade.