Mike Zhou-Beckenridge's family believes his step-father cared too much for him to kill him by driving them both off a cliff in 2015.

He and John Beckenridge disappeared in March 2015, their car found crashed at the bottom of a steep cliff in the Catlins.

A coroner's hearing on Monday heard opposing views from police, who called on the coroner to declare the pair dead, and the family adamant they were alive and in hiding overseas.

Mike's family hired private investigator Mark Templeman to look into the case, and he told the coroner they believed Beckenridge took steps to make it look like a murder-suicide to put people off their track.

Templeman said this included texts messages which were the last known contact with John Beckenridge.

"Essentially these texts are suicide notes, and there are a number of them. It is very hard to believe that he hasn't text or sent a suicide note to his very best friend in that group of people."

Templeman said the family believed the clifftop site the car went off was carefully chosen because Beckenridge knew it would be very difficult to check for bodies in a car crashed into the ocean at that spot.

"We believe that if someone was intent on driving off a cliff to kill themselves they could have done it anywhere along that cliff face, as you would be killed on impact anyway whether that was on the rocks, beach, rocky beach or water."

Templeman also said the family believed Beckenridge cared too much for Mike, then aged 11, to make him watch as his step-father placed a stake to aim for as they went off the cliff, sent a series of farewell texts, then plummeted off the edge.

He said the family believed they got on an ocean-going yacht or some similar sort of transportation and left the country.

"Mike's mother Fiona Lu, now Russell, believes that when Mike is no longer under the influence of John Beckenridge he will reach out to her. And she further believes that John cared too much for Mike to hurt him."

Police lawyer Deirdre Elsmore said they believed it may be Beckenridge's hatred for his ex-wife that drove him to kill himself and Mike.

"John's attitude, his absolute white hatred and feelings of intense betrayal towards Fiona - you might be left wondering that it is no coincidence that the two have died in circumstances that have left Fiona wondering if they are dead or alive," she said, "whether that is what might have been the intention, the ultimate punishment for her betrayal."

She said the police view was that Beckenridge lacked the ability to successfully fake a murder-suicide, and the pair were dead.

"The evidence of the experts, the evidence of the scene, the evidence of this extensive police inquiry, supports a conclusion that John Beckenridge at the time of this disappearance lacked the funds, he lacked the support of associates, and the police would submit he also lacked the clarity of mind to plan an extremely difficult, complicated escape from the Catlins area."

Police also said Beckenridge cared for his step-son but their view was his love, a belief the boy was suicidal, and Beckenridge's spiralling mental outlook led him to decide the best option was for the pair to die together.

Main image (RNZ/Ian Telfer): Police at the clifftop of where the Beckenridge car was found in the Catlins, Southland. No human remains were found.