It's a very good question Mani. I've thought a lot about it. My gut feeling and it's pure speculation is that she was placed there as its close to the Brisbane River and also perhaps because it might have been the most convenient place to dump a body in that area - even though a distance from the house. Often a body is easier found when buried. In water it's a little trickier but in a creek that is quite snaggy it's risky. The water also makes a difference to decomposition and trace evidence - but that's dependent on when the body is found. I think it seems apparent finding her was needed for the insurance money but I think GBC hoped she'd make it out into the river as:
1. More decomposition.
2. Further away reducing the likelihood he did it that night
3. Less chance for evidence and of course accurately determining cause of death - meaning it opens up the argument for suicide or abduction.
I personally think GBC wouldn't plead guilty as he's hoping the circumstantial case is weak and he'll get off via reasonable doubt. As Allison was found (and I think he was surprised perhaps by that) his MMO (means, motive, opportunity) are more established perhaps.
The creek is interesting. I know it but not well. The scout grounds is interesting also. One would have to think he knew that area reasonably well. As for where she ended up and started? Don't know. Lots of rain had to have played a part, and I don't think that was a factor considered by the dumper.
Just don't know - like the rest of us
plenty of great ideas on here though. I think surveyors being seen there is important as someone reported on here.