Southern police are playing a waiting game, as they prepare to remove the vehicle owned by the step-father of a missing Invercargill boy, from the Curio Bay surf.

Police now believe 11-year-old Mike Zhao-Beckenridge is dead, having been killed in a murder suicide by his step-father John Beckenridge.

They've concluded the pair were in the car when it was driven at high speed into the sea.

A police spokesman says, they're now waiting for favourable conditions, so they can winch the vehicle out of the water.