Not sure what you mean by technically a Saturday? Is that because the accused didn't sleep and was still partying from Saturday? It's alleged Samantha was murdered at around 8am on Sunday morning.
I don't know if I'd be travelling too far out of an area I'm quite familiar with to dispose of a body if it were me. Far too much in the way of possible CCTV and digital footprints, especially if you need your sat nav to get you home again from some random spot. Not to mention potential witnesses who saw your car. The area, which I think the accused knew well due to his outdoor lifestyle, potential for hiding from police on the back roads after benders, and growing up there, is littered with dense bushland areas and bodies of water. Plenty of locations to dump a body, including the good old mine shafts. It's like searching for a needle in a haystack,and I think he just got lucky so far. Or unlucky, whatever you want to call it given he is now in remand.
My theory is this: What started out as an accident or altercation turned to murder, likely by an instant decision fuelled by drink, drugs and/or anger, and the fear of this destroying his life, especially as he'd come across the radar of police before (his pending motor vehicle case). He probably had alcohol and drugs in his system and panicked, because that could be the nail in his coffin. Fighting one drink/drugs/reckless driving charge, OK. Fighting two, where someone possibly could have died, game over. Then the "Oh &^$#" moment. He either finds a way to dispose of her body very close to where she died, or gets her out of sight and into his vehicle, and madly thinks of what to do next. May even drive around areas he knows well, looking and thinking as he goes. Perhaps he drove through or disposed of her body in the Elsworth Street area that police became interested in.
At some point, knowing time was ticking with a body in the back, and possibly having to account for his whereabouts, he chooses a location. My money is on a body of water somewhere relatively nearby. No weighing her down, just tossing/dragging and hoping for the best, much like the phone. Job done, he goes home to shower, change, maybe sleep, try to go on like normal, or meet his obligations that day. Perhaps he took her ear buds, phone and watch, or a combination of these and worried about what to do with those small objects later. Maybe Samantha had her phone out and it was the only thing that was flung from her body during the murder.
After some careful thinking, and/or at a time that suited him better, he took the phone and threw it into a nice large nearby dam on his travels somewhere, or just a random dam he knew about. Perhaps other nearby dams may hold earbuds, a watch and a murder weapon? Everything disposed of in his mind, and he went about his daily life until cops got him and here we are.
All my imagination and thoughts only.