Yes, tonners were from memory 2660kg for V8. Tonner springs came as part of the Outback Pack special vehicle package on ute and van, I think they were also part of the optional Gross Load package as well. I can't recall ever seeing an increase in load capacity with them though, just handled a constant load better eg camper body on a ute. Remember a tonner's GVW was not just rear springs. It was also fully welded 6" rims, heat treated stubs, bigger tailshaft and unis, thicker axles, wider rear wheel bearing retainers, chassis gusseting behind the cab and light truck tyres.
The tonner is the only one you can fiddle with the towing capacity under NSW rules, as GVW is governed by the weight of the tray, so a steel tray could see a tonner weigh 1700kg. If you were to tow heavy loads regularly it'd be a good idea to place Tare and Gross kg on the side of the chassis or tray. A properly setup tonner is a good tow vehicle if you use a Chev or Holden 350ci engine with 3L80 or 4L80E box (TH400's), 3.08 or 2.78 diff with 15" light truck tyes and power steer they'll tow all day. You really need an EFI engine though or they start to get thirsty!

This all assumes GMH never published proper max towing mass for these.