The guy from the city Water Wise department came out and looked at the leak in the front - he thinks it's in the same place I do, and he tested a couple of things.
After I'd cut the grass, I dug out the wet soil and the hole quickly filled up with water. I couldn't get much deeper, but the water was flowing, so I knew I was close.
Vic called because the van wouldn't start - it'd been sitting in the garage for a few weeks without starting or charging, so this isn't unexpected :o( I went and jumped it from the car - just leave the car "running" so the big battery'll top up the little 12v one.
When we got home we agreed to turn the water off (it's leaking almost a gallon a minute now) overnight to stop pissing water into the ground, and to hopefully dry out the hole.