Oldy,mate I've given up looking for isolated tracts of land so myself and my son can ride in peace for a few hours.
Being in the building trade I often got to work in some isolated new housing estates.I'd load the bikes on for a ride and head off to these places on weekends.The last few times I had gone out,I'd found most of the places were either; overrun with dickheads behaving and riding dangerously,the areas were guarded by security,or the police aren't far away from dropping by,when there is that many bikes making noise.The last place I rode on public land was more than a year ago,near Blunder Crk off Johnstons Rd near the Greenbank Army Range,a 45 min drive,there were more nobs on bikes,than knobs on yer tyres that day.
What I've been doing is heading off to the bike parks,sure there are some fools there,but the places are large enough to find your own tracks to blast off along.
As evo550 wrote,Kenilworth on the Sunny Coast is apparently a great spot to ride(no I haven't ridden it,a mate of mine has and he reckoned it was brilliant),you can camp there too.It took me 2 and a bit hours to drive from Southport to Conondale,to attend CD6 with a ute load of bikes and camping equipment.Kenilworth is only a bit further up the road from Conondale.
Due to the nature of my job I can plan a weekend here and there to get out to the various parks around the place,except Manar Pk,its near 400ks away.There are also a few motor bike parks over the border that have camping areas or even have cabins set up,so you can sleep in some comfort.And they aren't a long trek by vehicle.
I know you're talking about weekend blasts somewhere nearby with the family and not going off on a holiday.Sadly though,these little back paddocks are being either built out by developers or,overrun,or just unsuitable for a family day out.
Cheers,Mark.