Two children who died after they were found in the Swan River on New Year's Eve were a four-year-old girl and a six-year-old boy.
The pair, from separate families, was spotted in the water at a popular family picnic spot in Burswood, Perth, about 5.25pm on Sunday.
They were unresponsive when members of the public pulled them from the river, Western Australian police said.
Acting Commander Mark Longman said it was a tragic incident.
"Two very young people have lost their lives," he told reporters on Monday.
"We can only begin to imagine the trauma and suffering that those (families) will be experiencing at this time."
Nine paramedic crews responded to the emergency and police cordoned off the area, about 3km east of the city.
The children were taken to Perth Children's Hospital in a critical condition but could not be revived.
The children's families were known to each other.
The incident happened near where revellers had gathered for New Year's Eve celebrations and close to a children's playground.
Acting Commander Longman said specialist support staff were helping the families.
"Our thoughts and prayers are with the family and those people that were involved," he said.
There have been 34 drownings across Australia since December 1, five more than the same time last year.
Lifesavers have urged families to take care when they are swimming.
"Tragically that period between Christmas and 2nd January is when the most people drown," Royal Life Saving Chief Executive Officer Justin Scarr said.
"It is critically important that an adult is actively supervising children at all times if you are near water."
Coronial Investigation Squad officers have taken carriage of the investigation.
