Police said Wednesday a boy and a girl, aged 10 and 12, were able to "extricate themselves", in what Brian Codd, Assistant Commissioner for Southeast Region of Queensland Police Service, described as "almost a miracle."
Previously, police had said the survivors were two girls.
Six people were riding on a "flume" at the Thunder River Rapids Ride at Dreamworld theme park in Queensland when the malfunction occurred.
Their flume flipped after it came in contact with another carriage towards the end of the ride, tossing some of the passengers onto the conveyor belt, where they were caught in the machinery, Codd said on Wednesday morning.
He said the two children are traumatized and being cared for by their family, adding they were lucky to be alive.
Two men, ages 35 and 32, and two women, ages 42 and 38, were pronounced dead by police following the incident.
Police were called to the site after reports that "a number of people had been injured by a conveyor belt."
Gavin Fuller, acting senior operations supervisor for Queensland Ambulance Service, said the victims had sustained injuries that were not survivable, adding that "a number" of staff from the ambulance service had been "deeply affected by this terrible incident."
Inspector Tod Reid of the Queensland Police told reporters that an investigation is under way and police are trying to contact the victims' families.
Theme park closed
Dreamworld, located in Gold Coast, about 45 minutes from Brisbane, is the largest theme park in Australia. It includes animal attractions and more than 50 rides, according to its owner, Ardent Leisure.
In a statement,
company secretary Alan Shedden said: "Dreamworld's focus and priority is with the families of those involved in this tragedy."
Craig Davidson, the park's CEO, announced its closure at a news conference following the accident.
"At this stage, the park is closed and Dreamworld is working closely with police and emergency services to establish the facts around the incident. We are deeply shocked and saddened by this and our hearts and our thoughts go to the families involved and to their loved ones," Davidson said.
Billed as a family ride, the Thunder River Rapids Ride has had problems in the past, according to a report
by CNN affiliate Seven News.
Condolences and shock
US Olympian Matthew Centrowitz tweeted that he had just stepped off the ride when he learned that there were injuries.
"Dude! Just got off a ride 15 mins ago that has 1 person seriously injured & 2 others trapped. #Dreamworld," the track star's tweet read. The death toll was updated after Centrowitz' tweet posted.
Australian Prime Minister Malcolm Turnbull said he is "very saddened" by the "tragic accident."
"Our thoughts and prayers are with the families of those who lost their lives. This is a very sad day, and we trust there will be a thorough investigation into the causes of this accident over the days to follow," said a statement on his official website.
Opposition Labor party leader Bill Shorten tweeted his condolences to the families of those affected by the incident.
"Heartbreaking news from Dreamworld. Sending our love to the families of those who lost their lives in this devastating tragedy," he wrote.
Gold Coast Mayor Tom Tate said that incident marked "a very sad day for our city."
"Our thoughts are with the families of those affected and the emergency staff in attendance.
"I urge everyone to show the respect needed as the relevant authorities undertake investigations in to what has occurred. A truly sad day for all," Tate added.
Opened to the public on December 15, 1981, Dreamworld attracts around 1.8 million visitors each year.
Ardent Leisure also owns another theme park, WhiteWater World, as well as the Skypoint observation deck, d'Albora Marinas, AMF and Kingpin Bowling and Goodlife Health Clubs.