now that the prosecution and defence arguments have concluded, i'm going to put forward a theory that at first most will dismiss but bear with me.
Obviously not a common cause of death in qld, much less in april but nevertheless i feel it is worthy of investigation.
From my admittedly limited research, hypothermia starts to become an issue around 60 degrees fahrenheit (15.5 degrees celsius). On the evening of the 19th of april/morning of the 20th the minimum temperature observed at brisbane airport was 17 degrees however as most brisbanites know, temperatures are routinely much, much colder in the western suburbs and ipswich.
Finding detailed climate information for a date more than 2 years ago is difficult outside of the major weather stations but with a little bit of trawling through the bowels of the internet it appears that the minimums in the general vicinity of where allison's body was found were anywhere between 13-16 degrees that night, with most around the 15 degree mark (there are lots of colder pockets/frost hollows in the area so it's difficult to determine exactly).
15 degrees is going to be chilly but not harmful to a healthy, non elderly person who is suitably dressed, however this is measured at some distance above the ground and allison was found on the ground.
The terrestrial temperature (the temperature immediately above the ground) is usually measurably cooler than the recorded air temperature. Depending on other variables, primarily wind, the difference can be significant. This is why we see frosts when the air temperature is well above freezing. On a windless night, the terrestrial temperature is routinely 5-8 degrees cooler from my observations. Where it is windy, the difference is usually much smaller, maybe 1-3 degrees.
Allison was found lying/slumped on the ground. Assuming 15 degrees (and it may well have been a couple of degrees cooler at that exact location) with a terrestrial temp of 6 degrees lower (as there was virtually no wind recorded at amberley during the relevant period), she has now been exposed to single digit temperatures for at least part of the evening/early morning. This slips under the 50 degree fahrenheit mark which seems to be significant when assessing the risk of hypothermia but again a healthy adult should survive the night, albeit uncomfortably.
However what if alison's clothing had become wet for some reason? Wet clothing massively increases the risk of hypothermia, something that i've personally experienced when climbing in colder climates. Dry clothing, no worry in the world but as soon as your clothing becomes the slightest bit damp, things can get serious very quickly. There were localised showers in the area at the time, she could have been dripping in sweat after running vigorously, there may well have been a dew, she could have collected the moisture as she was brushing through foliage or indeed she could have become soaked if she had jumped or fallen in the water for whatever reason.
9-10 degree temperatures with wet clothing is not a good recipe. I haven't come across any detailed research on this exact scenario, most deal with total immersion of the body in water but a person will generally die within a couple of hours of being in 9 degree water. Although obviously wet clothing isn't quite as serious as total immersion, you can start to understand the seriousness.
On top of this it appears that antidepressants/sedatives and the like can adversely affect the ability of a person to ward off hypothermia.
One of the paradoxical reactions to hypothermia is the compulsion to undress oneself. Often perished mountaineers are found in various stages of undress, could it possibly be that allison was attempting to remove her jumper for this very reason? Could she have sought the refuge of the bridge in a confused attempt to seek shelter?
A number of suppositions have to be made for this to be a plausible scenario (allison would have had to have been at ground level for an extended period, either injured or exhausted) but it's something that i would have explored further. And perhaps it was and was debunked early on in the piece, nevertheless it's possible in my opinion, albeit not likely.