CIRCUMSTANCES: "The Airtractor, VH-OMC, and an Ayres SR2 aircraft were operating in company. The spraying operation had been completed, and both aircraft were loaded with water in order to flush the hoppers and spray equipment during the return flight to St George. After departing the strip, the aircraft deposited the water onto a disused field and then headed for St George, flying at approximately 50 feet above ground level and about 30 metres apart. The pilot of the Ayres S2R saw VH-OMC, which was on his left, suddenly pull up into a climb. He then lost the aircraft from view but moments later saw a cloud of dust rising where it had impacted. Ground witnesses saw both aircraft flying level, approximately line abreast, when the Airtractor suddenly pulled up steeply, as if to make a turning manoeuvre at the end of a spray run. However, at the top of the pull up the aircraft became inverted and fell to the ground. An inspection of the wreckage revealed no mechanical defects which might have contributed to the accident. The engine was operating at impact."