The mother of missing Invercargill schoolboy Mike Zhao-Beckenridge has taken legal action to find if her son is a beneficiary of his estranged stepfather's house in Queenstown.

John Beckenridge and Mike, aged 11, have been missing since March 13 and there is suspicion that Beckenridge had committed murder-suicide.

A High Court decision has been made after Fiona Lu, Mike's mother and Beckenridge's former partner, sought a court order to appoint Public Trust as the trustee for the Ozprey Trust, substituting Beckenridge.

Beckenridge's friend Ulf Henricson was the second trustee for the trust, which owned a Lake Hayes property where Mike had lived with Beckenridge until a Family Court decision to move the boy to Invercargill to be with his mother.

The house has a rateable value of $700,000.

The court finding said Lu was unaware of the terms of the trust deed but believed her son may be a beneficiary.

Lu made a claim in the Family Court against the trust, through the Property (Relationships) Act 1976, after an email sent to her solicitors by Henricson said Mike had now been excluded as a beneficiary.

However, Judge Robert Osborne found a document Henricson attached contained "no evidence of execution or operative provisions such as might establish an effective exclusion".

Osborne ruled to have Public Trust appointed to represent Beckenridge in his capacity as trustee.

A spokesman for Lu declined to comment on the case saying he did not want to jeopardise court proceedings.

A case management conference has been set for August 18.

Beckenridge broke a Family Court parenting order, picking Mike up from James Hargest College's junior school.

On March 22 Beckenridge's car was found in the sea near Curio Bay, having gone over a cliff.

When police pulled the car from the water on May 6 they were unable to confirm if the pair had been in it when it entered the water.

Border alerts in place had not been triggered, despite reports authorities were investigating a sighting in Papua New Guinea earlier in the year.