Dads Tractor is correct.

I lived and worked up there for 14 odd years and my acreage when we did the compulsory rural thing was just after the Stuart Hwy went from 110 to Open.
I had a Courier business for a while that used to do a lot of Darwin to Katherine jobs and my Cousin owned a Trucking Company that used to do hi-speed freight between Adelaide-Darwin back before the train went all the way thru.
He was running Volvo Prime Movers pulling singles with parcels etc so light on and they used to cruise on 140 KPH
The majority of the road verge is cleared back 50 metres or so and south of Katherine you can see for miles ahead

I mention the above purely to illustrate we spent a LOT of time on the roads and an open speed limit up there makes a lot of sense.

I also believe the speed limit from Norseman to Ceduna (or maybe the whistle stops west of Ceduna) should be 140 as well.
Almost every car on the market these days is safe at those speeds and everywhere you look we are told fatigue is the biggest factor so why not let people get a boogie on.