I have a Fisher and Paykel E442B upside down, which is a very similar model, and I just fixed the same problem (temperature was around -4C even though I set the fridge temperature as warm as possible).
The cause of my problem was excessive ice build up on the side of the evaporator coil (which is in the back of the freezer compartment behind the rear plastic panel). I had the same problem around a year or so ago, so I expect it to be a regular occurence. To defrost the coil, I removed all the drawers in the freezer, and the plastic insert by pulling out the bottom of the panel out and rotating it upwards until it released at the top. There are a couple of snap-in tabs holding the panel in, so with a bit of pulling they released. The tabs are either side of the slot at the bottom of the panel.
I then supported the panel horizontally on the top shelf so that I didn't have to disconnect the thermostat wire and the circulating fan wire.
With a small fan heater set on low, aimed roughly at the ice buildup on the left hand side of the evaporator coil, I defrosted the ice built up. This took around 2 hours or so. Then put it all back together and it worked fine.
I'm not sure why the ice build up on the side caused the problem, but presumably it interferes with the proper flow of cold air to the fridge compartment.
If you don't seem to have ice build up like I did, then it could be a thermostat problem, or a fan problem (a symptom of which would be fridge being colder at the bottom than at the top).