Four-year-old Cleo Smith has been found alive and well, more than two weeks after she went missing at a campsite on Western Australia's northwest coast.

Police confirmed they rescued her from a locked house in Carnarvon about 1am on Wednesday.

"One of the officers picked her up into his arms and asked her 'what's your name?'" Deputy Commissioner Col Blanch said in a statement.

"She said - 'My name is Cleo.'"

The girl is well and has been reunited with her parents.

A man from Carnarvon is in custody being questioned by police.

Watch: Cleo Smith's mother and stepfather speak about their missing daughter for the first time. Post continues below.

Cleo Smith, four, was on a holiday with her family at the Blowholes campsite in Macleod, north of Carnarvon in Western Australia when she was reported missing on the Saturday morning of October 16. She was last seen wearing a pink and purple one-piece sleepsuit.

Pleading for help at the time, her mother Ellie posted on social media writing: "It's been over 24 hours since I last seen the sparkle in my little girls' eyes! Please help me find her."

Cleo has been found.

Cleo was found "alive and well" in a Carnarvon house at 1am on Wednesday morning, WA police said.

WA police officers "broke their way into a locked house" and found the girl in one of the rooms.

She has since been reunited with her parents.

BREAKING: CLEO SMITH has been found ALIVE. Detectives in Carnarvon found her in a locked house at 1am. @7NewsAustralia pic.twitter.com/vkGWwUnhAH

— Ben Downie (@Ben_Downie)

A 36-year-old man from Carnarvon is in custody and is currently being questioned by detectives. Police say he has no connection to the family. No charges have been laid.

"This is the outcome we all hoped and prayed for," said Deputy Commissioner Col Blanch.

"It’s the outcome we’ve achieved because of some incredible police work.

"I want to thank Cleo's parents, the Western Australian community and the many volunteers.

"And of course, I want to thank my colleagues in the Western Australia Police Force."

Video footage has since been shared of the moment Cleo Smith was rescued, with the detective on site telling the four-year-old, “We’re going to take you to see your mummy and daddy.”

“We’re going to take you to see your mummy and daddy.” This is the moment Cleo Smith was rescued by Western Australia Police, 18 days after the four-year-old went missing. DETAILS: https://t.co/PPhJTfqUkR #9News pic.twitter.com/2FHDPctJzx

— 9News Australia (@9NewsAUS)

Deputy Commissioner Blanch said police would have more information to share later on Wednesday.

"For now - Welcome home Cleo," he said.

Prime Minister Scott Morrison tweeted his relief at the news.

What wonderful, relieving news. Cleo Smith has been found and is home safe and sound. Our prayers answered. Thank you to the many police officers involved in finding Cleo and supporting her family.

— Scott Morrison (@ScottMorrisonMP)

NSW Police Commissioner Mick Fuller praised the police work of his WA counterparts.

"I thought the chances of finding her alive were so slim. West Australian Police never gave up on Cleo, it was good old fashion police work that resulted in her being found alive," he told2GB'sBen Fordham.

A police tip-off.

Commissioner Chris Dawson confirmed a tip-off led police to the house in Carnarvon.

"There was some information we followed up on. We had been following, you know a lot of the forensic leads and it led us to a particular house," he said, according to the ABC.

He added that police "put everything we had" into the investigation.

"Our homicide squad trained, experienced investigators/detectives, our analysts, our technical people, drone pilots, air-wing, the whole lot," Commissioner Dawson said.

"We mounted our general duties police who did a tremendous job within minutes of arriving, the first officers in the scene, within minutes, declared it a forensic scene and sealed it off which was just really, really good policing. From that moment on, it's just been a really dedicated team of well over 100 on the scene itself and we were prepared for a long hall but we're so, so pleased."

Cleo's mother is yet to make a statement, however, she did express her relief on Instagram, writing, "Our family is whole again".

Detectives label it an "opportunistic" crime.

Earlier this week, detectives believed it was "more than likely" that Cleo Smith was abducted in an "opportunistic type event".

As the search for the young girl entered day 17 on Tuesday, November 2, lead investigator Detective Superintendent Rod Wilde said it was likely Cleo was targeted in a short period.

"It’s more than likely an opportunistic type event," he told 6PR radio.

"We know they got there ... on the Friday night. It was getting dark and so there would have been limited opportunity for people to observe Cleo at that time."

Superintendent Wilde said police confirmed Cleo was at the campsite via video footage on a parent’s phone that had a geolocation tag, as well as audio of her voice on CCTV at a nearby shack.

"We've got that CCTV but there's also some video footage that we've retrieved off the parent’s phone that puts the fact that the family was there and Cleo was at the campsite," he said.

"We've got some independent forensic material that corroborates that fact."

The search.

An air, land and sea search got underway with help from the SES and community volunteers soon after Cleo was reported missing.

"We are greatly concerned for the safety of Cleo and we aren't leaving anything to chance or ruling anything out," Inspector Munday said the afternoon after her disappearance.

"We are going as hard as we can for as long as we can."

At the time, Munday said he was "fairly confident" police were able to get the identities of all the people who were in the campsite at the time she disappeared, and gather information from dashcams and CCTV cameras in the area to "paint a picture of who was around here".

On the following Monday, police released additional photos, including a photo of Cleo's sleeping bag and her pink and purple sleepsuit.

Have you seen four-year-old Cleo Smith? Cleo was last seen sleeping in a red and black sleeping bag and was wearing a pink/purple one-piece sleep-suit with a blue and yellow pattern. Read more here: https://t.co/8iyvaTusLA pic.twitter.com/46wYK6ZLem

— WA Police Force (@WA_Police)

Monday also brought "grave fears" for the missing four-year-old after a cold front hit the area, bringing heavy winds and rain.

"Every hour that goes by with a four-year-old out in this environment, there’s obviously limited water supply, it is warm, very limited shelter. It is very, very concerning," Munday said.

"That’s why we do hold grave fears for Cleo at this time."

The next day, police called off the marine and sea-based search for Cleo and turned their attention to remote coastal shacks close to where she was last seen. A police helicopter and drones were also deployed as part of an aerial search.

Munday would not comment on reports a car had been heard speeding from the campsite in the early hours of the morning.

A $1 million reward offered.

On October 21, the WA government announced they were offering a reward of up to $1 million for information that led to the location of the missing four-year-old or to an arrest or conviction of anyone involved in her disappearance.

"I urge anyone who has any knowledge of the location of Cleo, please provide that information to police and ensure that we can provide some certainty and information to Cleo's loved ones," WA Premier Mark McGowan said at a press conference.