The mother of two children killed in the Redcliffe plane crash has revealed she was wrapping Christmas presents for them when police knocked on her door to tell her they had been killed. 

Lucas Mocanu, 9, and his sister Lavinia, 10, their father Cris Mocanu, 41, and pilot Robert "Roy" Watterson, 67, were killed when the plane they were in crashed shortly after taking off on Sunday. 

Catherine Di Blasio told The Courier-Mail said she had no idea they were going on a flight, which was a surprise for her son's birthday. 

Mocanu, who worked at Watterson's company, had asked his boss if he could take the children on a flight. 

"He (Lucas) had a recent birthday in November and his dad told them that it's a surprise, a present, but all he said it was an experience that you will love," Di Blasio said. "Up to that morning the kids didn't even know." 

Di Blasio, who was separated from Mocanu, said they were expected back at lunch but by 4pm began ringing his mobile phone which kept giving "out of service" responses. 

"Then I thought they were ringing my doorbell but when I opened the door there was three child protection service detectives and they told me the bad news," she said. "Even the guy had trouble telling me, he didn't know which words to use, it was pretty horrible. 

"When they came to my house, the detectives, I was wrapping the kids' Christmas presents." 

Di Blasio said her last words to her children when she dropped them off with their father were: "Have fun guys, I love you." 

"My son turned around and said 'love you too mum' and they left." 

Di Blasio, who moved to Australia 14 years ago with Mocanu, said their families were planning to travel from Montreal in Canada to be by her side. 

Catherine Di Blasio and her children Lucas and Livinia, who died with their father Cris Mocanu (R) in the tragic crash. 

Watterson was a prominent millionaire businessman from the Moreton Bay area. 

Queensland Police confirmed in Sunday evening that a 41-year-old Brisbane man, his 10-year-old daughter and 9-year-old son also died in the crash. 

"The families became aware when they knew the aircraft hadn't returned on site and they began looking at social media posts," Inspector Craig White said. 

"The family are deeply traumatised, as you can expect." 

The plane, a Rockwell Commander 114 with registration VH-WMM, was owned by Watterson. It's believed to have been taken on a prearranged joy flight. It is understood it was an unpaid flight. 

Speaking to media on Sunday afternoon, Inspector White said the aircraft was seen to disappear behind two mangroves, where the wreck was spotted by another plane shortly after. 

The pilot has been identified as Robert "Roy" Watterson. Photo / Facebook 

Police recovered the bodies of four people at 12pm and authorities will continue their investigation into the cause of the accident. 

Chief commissioner of the Australian Transport Safety Bureau (ATSB), Angus Mitchell, said aircraft operation and maintenance teams from Brisbane and Canberra will be arriving at the scene to determine the nature of the crash. 

He also called on any potential witnesses to come forward to help authorities with the investigation. 

"Early reports are it looks as though it was not long after takeoff, however we'll need to confirm that with air traffic control," said Mitchell, adding the ATSB will release a preliminary report in six to eight weeks. 

A light plane crashed into the water near Brisbane on Sunday, killing four. Photo / 7 News 

The drying mangrove is expected to make the recovery of the wreckage quite difficult. 

The Civil Aviation Safety Authority CASA confirmed to news.com.au that the aircraft was a 1977 Rockwell Commander; a four-seater, single engine plane. 

Queensland Police also confirmed that a multi-agency operation would be involved in the investigation. 

"A multi-agency operation is underway with Queensland Water Police officers and divers along with the Forensic Crash Unit assisting other agencies, including the Australian Transport Safety Bureau, as investigations continue," a statement read. 

Speaking to media earlier today, Police Commissioner Katarina Carroll said the plane was in a difficult position for rescuers to reach. 

"The plane is in a very, very difficult position in the wetland area and we currently have police and divers travelling to that area," she said. 

The suspected location where the plane crashed. Photo / FlightAware 

According to FlightAware, the plane went down near Redcliffe, off the coast of the Kippa-Ring Conservation Reserve. 

Coast Guard, Australian Transport Safety Bureau and police boats flocked to the scene to help, but a spokeswoman for Queensland Police told news.com.au a specialist diver was needed to get inside the aircraft. 

The crash site is close to Redcliffe Airport, but it is not yet known how long the plane was in the air before it crashed, or what caused the incident. 

It's not known if the plane's pilot was part of the Redcliffe Aero Club, with a spokesman for the club declining to comment when contacted by news.com.au. 

- by Hannah Moore and Jessica Wang, news.com.au