Thats a tough one. Does it have to be bitumen rd?
You could go to Gawler, then Mallala rd to Mallala, then Balaklava, then Port Wakefield, after that I think your limited to just using the Copper Coast Hwy to Wallaroo.
I frequent Wallaroo often, few times per year, and would love to ride there from Adelaide, but to be honest the Copper Coast Hwy between Port Wakefield and Kadina might be hary on a bike, especially during a holiday period.
Hopefully someone else has a better idea than I have, if not try using nearmaps so you have an idea of road surface and width.
Chuck some 25s on @ 85 psi.... whole new world. Bunch of 'old' roads up there.
Follow the railway if your not in a hurry.The roads Adelaide/williamstown/ balaclava are some of the best around.
Consider going to Balaklava, Nantawarra, Lochiel, Ninnes and coming out just north of Paskeville at the field days site if you want to go around Port Wakefield. It will add a few extra km's but the traffic will be minimal. The road from Nantawarra to Lochiel is on the highway but only for about 5kms and it has a pretty generous shoulder to ride on, but you still have road trains etc.
Ive riden to Yorkes and Wallaroo a few times. Pt Wakefield Rd is the worst road Ive cycled on earth in terms of narrow shoulder and redneck speeding traffic.
Best route would be Mallala-Balaklava-Kadina etc.