Two victims were thrown from a raft on the Thunder River Rapids ride at Dreamworld on Queensland`s Gold Coast, while two others were trapped inside, officials said.
The park in Coomera has been closed and an investigation is under way.
Dreamworld bills itself as Australia`s biggest theme park with more than 50 rides and attractions.
The Thunder River Rapids ride whisks visitors in circular rafts along a fast-moving artificial river.
Dreamworld describes it as a "moderate thrill" attraction.
Queensland Ambulance spokesman Gavin Fuller blamed a "malfunction" for the accident.
The victims have not been named but are reported to be two women aged 32 and 42 and two men aged 38 and 35.
Australia`s 9News reported that they were possibly from the same family.
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10:13
Four people have been critically injured on a ride at the Dreamworld theme park on Australia`s Gold Coast, say police.
Details are unclear, but Queensland police said they were attending a "critical incident" at the park.
"There is nothing further we can confirm at this stage," they said in a Twitter statement.
Dreamworld bills itself as Australia`s biggest theme park with more than 50 rides and attractions.
The incident is believed to have happened on the Thunder River Rapids ride.
Currently Australia`s only river rapids ride, it was opened in 1986.
The theme park`s website says it allowed riders to travel at up to "45km/h through the turbulent rapids".
Dreamworld: Four killed on Australian theme park ride - VIDEO, UPDATED
25 October 2016
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Two men and two women have been killed on a ride at a theme park on Australia`s east coast.
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