Australian rules (from memory, so pleas excuse any errors). According to Oz AIP these are ICAO compliant.
May Descent below MDA:
1. Within the circling area for category: Cat A = 1.68nm, Cat B = 2.66nm, Cat C = ??, Cat D = 4.xx (?), Cat E = 5.xx (also ?)
2. Clear of cloud, in sight of the approach end of the runway, or lights or markings associated with the approach end of the RWY.
3. In sight of the ground or water
4. Visibility not less than that specified for circling.
5. IAS as specified for cirling for a/c category.
5. By day: Descent to not less than Obstacle Clearance Height (OCH)
For Cat A & B = 300' above highest obstacle in the circling area, Cat C&D = 400', Cat E = 500'
By Night: Maintain MDH until intercepting the normal descent profile for a landing approach to that runway. In this case descent would commence where ever the gradient meets the aircraft's flight path eg final, crosswind, downwind etc.
Someone please correct my lapses in memory!
Hope this helps.
T