The Cpl-rated commander was conducting a revalidation of the handling pilot’s class rating in a Cessna Cardinal RG. A flapless circuit to Denham’s Rwy 24 was flown and the handling pilot raised the undercarriage on the crosswind leg, just prior to turning downwind. Raising the undercarriage was not standard procedure in the aircraft when flying circuits, and the commander intended to tell the handing pilot to lower it again once the wheels had finished travelling, but a long period of radio communication from two aircraft ahead in the circuit distracted the handling pilot from lowering the undercarriage, and the commander from noticing that it had not been lowered. An aircraft joining the circuit overhead caused further distraction, and consequently the undercarriage was not lowered as part of downwind checks. A low sun angle and attention to trees on the flapless approach demanded greater attention than normal, and the landing was completed with the wheels still up. Maintenance inspection revealed that the undercarriage warning horn had not been working.