This is a looper caterpillar, so called because of the loop formed when it moves. Chomping its way through the tomatoes, a Chrysodeixis - either Chrysodeixis eriosoma or Chrysodeixis subsidens, family Noctuidae. Como NSW Australia, December 2011.
Chrysodeixis have 2 pairs of abdominal prolegs. When disturbed, the caterpillars move the front portion of their body about in wide sweeping arcs. GJ Goodyear, NSW Department of Agriculture Pest Bulletin 58, 1977.
Note that the looping action of this noctuid is different to the 'true' looping action of a geometrid caterpillar, which inches its way along by repeatedly stretching out the front of its body and grabbing on, followed by a loop of the body which brings up the rear end.
See E H Zeck's wonderful watercolour of a Green Looper.