Gemma Gaye Killeen, 22, is accused of killing her 22-month-old son after the toddler was found floating in the ocean off the marina's northern carpark just before 6pm yesterday.

Police today named the dead toddler as Te reringa Kayden Ashley Weter.

Officers were called to southern carpark at 5.40pm after Ms Killeen made a 000 call, saying the child was missing.

"The mother of the child, Gemma Killeen, claimed the child went missing in the carpark," Detective Inspector David Bryson said.

Police started a search for the child, but minutes later, the child was located by passersby in the sea.

"The members of the public commenced CPR on the child. St John Ambulance was called. The child was taken to hospital but was dead on arrival at the hospital," he said.

Major Crime Squad detectives charged Ms Killeen with murder after questioning her last night. She appeared in Joondalup Magistrates Court this morning and has been remanded in custody to reappear in Stirling Gardens Magistrates Court on December 8.

Det-Insp Bryson would not say if police believed the child had drowned or if he was already deceased when he was allegedly put in the sea.

A post mortem will be conducted on the child on Monday to determine precisely how he died.

Police today revealed that Ms Killeen had been seen in the northern carpark in her blue-green Holden Nova, registration 1BMR-294, when a light-coloured four-wheel-drive with a small runabout on the roof also pulled into the carpark. Ms Killeen was seen speaking with the driver of the 4WD.

Det-Insp Bryson urged the driver of the 4WD to contact police. In addition, detectives are want to speak with anyone who may have been in the northern carpark yesterday afternoon between 5pm and 6pm.

Det-Insp Bryson said police had seized closed circuit surveillance footage of the carpark, but declined to say if the vision showed the incident.

He said police would speak with Ms Killeen's partner as part of the investigation, but he declined to comment on the status of the relationship.