Two Australians including a 10-year-old girl have been killed in the Sri Lanka bomb blasts and another two have been injured.
Prime Minister Scott Morrison has confirmed the deaths, as the number of people believed to be killed in the attacks has risen to at least 290.
"Right now, Sri Lanka is grieving, Australia is grieving," Mr Morrison told reporters in Melbourne.
"Two Australians have lost their lives in this terrible massacre and my heart is full of grief for them and their families."
While Mr Morrison did not go into details, SBS reported on Monday the dead were an Australian mother and her 10-year-old daughter who were attending a church service in Negombo.
SBS said the mother, Manik Suriaaratchi, and her daughter Alexendria had returned to Sri Lanka several years ago to look after a family member, according to a friend of the family.
The prime minister confirmed another two Australians - both of whom were dual citizens - have been injured but are in a stable condition.
One has been treated for shrapnel wounds and the other a broken leg, with both receiving consular support.
Mr Morrison said it was too early to say whether more Australians could be among those who have been hurt.
"As the days pass, and the injured are treated, and some of them succumb to their wounds as well, we are seeing this massacre going from what was bad, very bad, to much, much worse," he said.
More than 500 people have been injured in the bomb blasts that ripped through three churches, three luxury hotels and a guesthouse on Easter Sunday.
The Australian High Commission in Sri Lanka has been working to determine the whereabouts of Australians and monitor the situation.
Australia's travel advice for Sri Lanka has been upgraded to "reconsider your need to travel" in the aftermath of the attack.
The Department of Foreign Affairs and Trade says Australians concerned for family or friends should try to contact them directly before contacting the consular emergency centre.
Labor leader Bill Shorten described the attacks as devastating.
"At a time like this, politics is not as important. At a time like this, we are all Australians," he told reporters in Townsville.
"Easter Sunday's senseless and barbaric murder of hundreds of citizens, including two Australians and 500 injured, is something we should all mourn and we do."
Mr Morrison and Foreign Minister Marise Payne have been in contact with Sri Lankan Prime Minister Ranil Wickremesinghe and Foreign Minister Tilak Marapana.
Australian Associated Press