The 35-year-old was swept out past the breakers when he and two other men came to the aid of the man in trouble at Miami Beach about 6pm on Thursday.
Acting Chief Superintendent Rhys Wildman says the swimmer was safely brought back to shore, but the rescuer was swept out into the ocean.
It was still half an hour off sunset and almost an hour from last light, but an extensive search failed to find him before dark.
Police combing the beaches in all-terrain vehicles and in boats offshore failed to find the missing man on Friday.
"Now, we still have not recovered that person or observed that person since," Chief Supt Wildman told reporters on Friday afternoon.
"We obviously have grave concerns for that individual, and it's a tragic tale where a member of the public has attempted to do the right thing and stepped into to save the life of another person, and unfortunately has found themselves in some difficulty."
He said the search would continue in coming days with police and other agencies trying to map currents, tides and winds to pinpoint where the man could be.
"If anyone sees anyone that they think is in distress ... then make contact with us," the officer said.
"And obviously if anyone finds the missing person, who may unfortunately wash up, then ring obviously triple zero straightaway."
Chief Supt Wildman said police have been in contact with the missing man's family in Queensland and interstate.
He said the two other men involved in the rescue and the swimmer are still coming to terms with the situation.
"At the time they were focused very much on recovering the initial male, who was in some difficulty," he said.
"Whilst that person was recovered, obviously as the situation unfolded, the full realisation is now coming to light.
"Obviously our thoughts are with them as well."
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