I think it must be some other issue. My DK35 has over 200 hrs and starts fine. The other day at about -5 C it started easily without glowing (I did not mean to skip that step). I've made the same mistake a few times around +5 or so and it starts instantly.
When you say it won't turn over, do you mean the starter does not turn the engine over the first time you try to start it? Or do you mean the starter works but the engine won't fire? If the former, it could be something to do with the safety stuff in the starting circuit. Such as the switches to make sure it is in Neutral, the clutch is in, and the PTO is off; or the relay they control.
I don't know the maximum time recommended for using glow plugs. My previous 30 year old Kubota
L275 was hard to start and I had to use the glow plugs at least 60 seconds. In cold weather I would hold it on 90 seconds, it would fire but not start, glow again for 30 seconds, fire but not start, glow again, etc.. Perhaps the glow plugs were weak. I had a radiator hose heater that helped in cold weather and if it was quite cold (-30 C or so), I would sometimes tarp the engine and put an electric heater under it for a couple of hours. That made a big difference. But I'm sure you won't be doing that in Queensland.