The two Australians killed have been identified as mother Manik Suriaaratchi and her daughter Alexendria. It is understood they were killed when a bomb went off at a church in the predominantly Catholic city of Negombo.

The news came as authorities found more bombs scattered around Sri Lanka on Monday. Eighty-seven bomb detonators were found in Colombo - at Colombo's main bus depot and at a garbage dump in the same area.

A van parked near a Sri Lankan church that was bombed on Easter Sunday also exploded on Monday when bomb squad officials were trying to defuse it. No injuries were immediately reported.

A night curfew went into effect at 8pm, the government announced.

Lucien Pereira said her friend Ms Suriaaratchi and Ms Suriaaratchi's husband Sudesh, who is believed to be safe, moved to Sri Lanka with their daughter in 2014.

"They are so beautiful, so helpful to everyone. They're just not like normal people, they are so good. I don't want to believe that we've lost them," Mr Pereira said.

Friends and loved ones paid tribute to the mother and daughter on social media, with Fahim Mawjood saying it was "really shocking to know that you are no longer here with us".

"You lost your lives on a holy day in a holy place. The sweet memories of Alex and yours will live in our hearts forever. May your souls rest in peace Manik and Alex," he wrote on Facebook.

Prime Minister Scott Morrison, who revealed the Australian deaths late on Monday afternoon, said one of the victims was a dual national and both had been living in Sri Lanka.

"Sri Lanka is grieving, Australia is grieving. Two Australians have lost their lives in this terrible massacre," Mr Morrison said. "My heart is full of grief for them and their families, as I'm sure all Australians' are."

Mr Morrison also said that two Australians were among the 500 injured - a woman in her mid-50s and a woman in her mid-to-late 20s. One has been treated for shrapnel wounds and the other has a broken leg, but the pair were in stable condition, Mr Morrison said. Both of the injured are Sri Lankan-born, dual-nationals and are being supported by Australian diplomats.

Mr Morrison said it was still too early to be sure other Australians had not been affected. If other dual nationals had been caught up in the blasts, they might not be immediately identified as Australians, as seemed to be the case with the two killed and two injured.

"As the days pass and the injured are treated, and some of them succumb to their wounds as well, we are seeing this massacre go from what was bad, very bad, to much, much worse," Mr Morrison said.

"It is just a terribly sad day. Sri Lanka will be just devastated by this, as I think all peace-loving peoples of the world are."

Citizens of at least nine countries killed

Of the 290 victims - the vast majority of whom were Sri Lankan citizens - officials said 39 were foreign tourists.

They included the children of Danish billionaire Anders Holch Povlsen, who owns international clothing chain Bestseller, is the largest private landowner in Britain and the largest single shareholder for the popular online fashion retailer Asos.

Povlsen and his family were visiting Sri Lanka for the Easter holiday, the BBC reported. Three of his four children were killed in the bombing attacks, a Bestseller spokesman told BBC.

"Unfortunately, we can confirm the reports," the spokesman said. "We ask you to respect the privacy of the family and we therefore have no further comments."

Sri Lanka's top diplomat in Britain said authorities know of eight British nationals killed in the bombings. Indian officials said five Indians died in the attacks, while two Chinese citizens were also killed in the blasts, according to state media.

The United States, Japan and Portugal have also confirmed their citizens were among the dead.

On Monday night, Sri Lankan authorities ordered a nationwide curfew for a second night in a row as investigations into the deadly attacks continued.

More than 1000 troops have been deployed in Colombo to ensure security, the army said.

Late on Monday, police conducting searches found 87 detonators at the city's main bus station, while a separate search uncovered explosive devices inside a van parked outside one of the churches targeted.

Efforts to defuse the devices resulted in a fresh explosion that caused momentary panic, but there were no reports of injuries.

Authorities had not identified any group as being behind the attacks, but two dozen suspects have been detained.

Sri Lanka's Prime Minister, Ranil Wickremesinghe, said "foreign links" to the attacks would be investigated and suggested authorities had received warnings but "not enough attention had been paid."

Cabinet spokesman Rajitha Senaratne pointed the finger at National Thowheeth Jamaath - a little-known group that experts said promotes Islamic terrorist ideology - but said "we do not believe these attacks were carried out by a group of people who were confined to this country.

"There was an international network without which these attacks could not have succeeded," Senaratne said.

No one has claimed responsibility yet and authorities have warned against speculating about their possible affiliations.

Sri Lanka endured regular terrorist attacks, including suicide bombings, during a bloody civil war between Tamil separatists and the Sri Lankan army which ended in 2009.

But the nature of Sunday's attacks, which targeted foreign tourists and Christians, was a shift from the violence that marked that three-decade long conflict.

Ariyananda Welianga, a Sri Lankan government forensic crime analyst told the Associated Press that the six near-simultaneous attacks on three churches and three hotels in and around Colombo were carried out by seven suicide bombers.

Analysis of body parts collected from the scenes showed the blasts were detonated by suicide bombers, he said.

Welianga said two people were involved in the attack at the Shangri-La hotel. One bomber each attacked the Cinnamon Grand hotel, the Kingsbury hotel, St Anthony's Shrine in the capital Colombo and St Sebastian's church at Negombo north of Colombo. There was also a blast at Zion Church in Batticaloa, a city in Sri Lanka's north-east.

The deadliest bombing appears to have been at St Sebastian's Church, Negombo, about 8.45 on Sunday morning. Police reported more than 100 bodies at Negombo's main hospital following the attack.

Two bombings hours later at a guesthouse and near an overpass on the outskirts of Colombo are still under investigation. Suspects detonated explosives at a safe house near the overpass blast, killing three officers.

Sri Lankan military also safely detonated a homemade bomb found near the departure gate at Colombo airport late on Sunday.

Sri Lankan authorities temporarily blocked social media services including Facebook, YouTube, WhatsApp, Instagram and Snapchat following the attacks in an apparent bid to stop the circulation of false information and reduce the risk of further violence.

The government imposed a similar ban on popular social media services for a week in March last year because of concerns they were being used to fan anti-Muslim violence in central Sri Lanka. About 7 per cent of Sri Lanka's population are Muslims.

with wires