Just to clarify, the purchasing of the sandbags direct quote is:
"Through electronic means, through GPS tracking, through CCTV tracking we were able to establish certain facts in relation to the vehicle he was driving and those movements, and also certain suspicious behaviour which escalated our scrutiny in relation to his involvement, or potential involvement. Those movements included on Thursday afternoon after a number of telephone conversations with the girl's mother, to purchase a number of sandbags, 20kg sandbags, from a hardware store, to fuel a boat, and then try and float that boat on the water at one of the docks in inner Sydney, that was unsuccessful because the boat was inoperable."
I don't read that as the mother purchased anything, only the accused.
They have said at tthis stage they don't have any evidence to support anything other than that he acted alone, but the investigation is ongoing.