Preparing for a new lawn

We have just had our section re-contoured by a bobcat. This involved scraping all the top off and re leveling. Unfortunately all the old turf and weeds that were buried are coming through. Is there a weedspray that I can apply that will kill the old grasses and weeds that are coming through and still in the ground but not leave a residue in the soil so that I can apply new grass seed. Thanks

The majority of weed killers are 'non-residual' or whatever the term is. Just read the label on the bottle and look for one that says words to that effect.

edit - i shouldn't have said 'the majority'. what i meant to say is there there are plenty of 'non-residual' weed killers available.

i've used Nature's Way before with some success. but grasses can be pretty hardy and will often come back even after spraying.
http://www.yates.co.nz/products/weed-control/gardens-paths-and-driveways-non-selective-control/natures-way-greenscape-concentrate/

cheap roundup will do the job.

Yup, I agree the "Just" brand from the warehouse has an equivalent to Roundup (exactly the same ingredients at half the price). Do a spray of that, and invest in a load of really good, fine topsoil before you do the lawn. Gives a nice base and healthy start to the new lawn. Incidently, we have had 3 new lawns over 17 years, by far the best was the spray on one. We did the prep, they came, sprayed it on, gave good instructions and watering plan, then came back to patch up any areas needing it. None did and its still going strong and looks beautiful.