A land, sea and air search for the woman was continuing, the spokesman said.
The woman was swimming with friends at Thornton Beach at the Daintree, 120km north of Cairns, Queensland when she vanished about 10.30pm last night local time.
Australian TV reports claimed the 46-year-old was heard to scream "a croc's got me", before she was pulled under the water.
Police said the woman and a friend, 47, were in waist-deep water when the croc struck.
"Her...friend tried to grab her and drag her to safety but she just wasn't able to do that," Senior Constable Russell Parker told ABC radio.
The two women were not locals and might not have been aware that the area was well known as a crocodile habitat, Parker said.
The friend was taken to the Mossman hospital, suffering shock and a graze.
Queensland Ambulance Service spokesman Neil Noble told ABC: "The report that we have from the surviving woman is that they felt a nudge and her partner started to scream and then was dragged into the water."
Warren Enstch, who represents the area in the Australian Parliament, said the beach was beside a creek where tourism operators run crocodile-spotting tours.
Enstch said the two tourists had to have seen plentiful crocodile warning signs in the region.
"If you go in swimming at 10 o'clock at night, you're going to get consumed," he said.
The search has resumed for the woman, with SES, land and water police scouring the bushland near Thornton Beach.
The case has echoes of a fatal croc attack near the Daintree River just before Christmas in 1985.
Beryl Wruck, 43, was taken by a 5m croc after a fateful decision to take a late-night dip in the shallows of Barratt Creek after an evening of dancing and drinking with friends.
More recently, 5-year-old Jeremy Doble died when he was attacked by a 4.3m crocodile in a swamp behind his Daintree River home.
He vanished in February 2009, with his seven-year-old brother telling police he saw a croc in the water moments after the boy disappeared.
RECENT CROCODILE ATTACKS IN QUEENSLAND
• May 29, 2016: A 46-year-old woman is snatched and dragged under water during a late-night swim with a friend at Thornton Beach in the Daintree National Park. Her friend tried in vain to save her.
• March, 2016: Cooktown sailor Graham Clark fended off a crocodile with a piece of wood after it latched onto the side of his boat at the town's harbour. The 70-year-old was woken by barking fox-terrier Laddie.
• December 2, 2015: Snorkelling off the coast of Lizard Island at night, Noosa man Mick Curwen was bitten on the arm by a 2.5m crocodile. He managed to shock the reptile by shining his torch in its eyes before frantically swimming to shore.
• April 13, 2015: An elderly golfer was attacked at a Port Douglas course owned by Clive Palmer. The 70-year-old man disturbed a crocodile in a waterway at the 11th hole of the Palmer Sea Reef Golf Course and suffered puncture wounds.
• March 9, 2011: A fisherman suffered a broken leg after being struck by a four-metre croc at Weipa in the Gulf of Carpentaria. The 28-year-old clung to mangroves before another fisherman was able to fight it off.
• February 5, 2011: A north Queensland man was swimming at a popular watering hole near Weipa with his two daughters when a crocodile latched onto his arm. He punched it several times in the head before it let go.
• February 8, 2009: Five-year-old Jeremy Doble was taken on the banks of the Daintree River in Cairns. He was playing with his older brother when he followed the family's dog into the water.
• September 30, 2008: Vietnam veteran Arthur Booker, 62, was killed while checking a crab pot on the Endeavour River near Cooktown. Two weeks later remains were found in the stomach of a 4.3m crocodile.
