Due to smoke haze conditions, the senior tower controller had advised other controlling agencies some hours previously that the tower could not exercise visual separation in the circuit area. Aircraft were being provided with radar separation for landing. When the two subject aircraft were approaching for a landing on runway 14, the slower aircraft was in front. That aircraft was asked to maintain its best possible speed and the following aircraft was slowed as much as possible. Separation reduced to below the radar limit when the aircraft were on final approach, with the second aircraft still unable to see the aircraft in front, about three miles ahead. Both aircraft landed safely. The approach controller had not taken sufficient action to ensure that the aircraft were adequately separated during the approach to land. FACTORS 1. Smoke haze reduced visibility. 2. The approach controller did not provide adequate separation to the landing aircraft.