Aussie Australian Plumbers or Builders older guys

Save me! In an early 90's set of small units joined side by side 4 in a row. Small court yards at rear, common area and driveways at the front, main road is to the side of the last unit. Can't find the sewer line!. Plumber has poked and dug, I have dug across the rear yard and the likely exit point at the front. No hatches anywhere, nothing on the street verge area. We're blind and there's no plans available on-line for these.