The total area of the sanctuary is 172.403sq.km and is stratified into 3 zones.
The core zone is a protected zone where human interference is strictly prohibited. This zone is in the eastern portion of the sanctuary and supports the prime wildlife habitats and dense vegetation having an extent of around 76.90 sq.km which comprises nearly 44.60% of the total area. The western boundary starts on the southern sanctuary boundary line at an elevation point of 1169m at Ambakallumottai and moves northwards through the boundary of Umayar touching the reservoir and further moves northwards through the reservoir boundary and stretches along the northern boundary of the reservoir till the point where Uruliyar falls into the reservoir, then moves northwards through the boundary of Uruliyar and touches the trekpath leading to Kannampallimedu and ends at the northern boundary of the sanctuary at Kannampallimedu. The core zone was constituted based on ecological considerations on account of rich biodiversity. The areas coming under this core zone consists of Pandimottai, Alwarkurichi, Umayar, Aruliyar, Narathar, Dharbhakulam etc. The major portion of west coast evergreen forest and subtropical hill forest are seen in this zone. Moreover, this zone consists of Southern hilltop evergreen forests, grasslands, reed breaks and few pure patches of Bentinckia condapana. This zone is comprised of undulating terrain. The main objective of constituting this core zone is to preserve it in as natural a condition as possible by providing full protection. All forms of biotic interference are strictly regulated except scientific studies contributing to better management and enhancement of the quality of biodiversity.
The buffer zone is located in the middle of the sanctuary sandwiched between the core zone in the east and tourism zone in the west. The buffer zone covers an area of about 68.40 sq.km. which is 40% of the total sanctuary area where in a major part of reservoir area is also included. The western boundary of buffer zone starts from the southern boundary of the sanctuary where the Rockwood estate meets with the sanctuary boundary and then moves northwards crossing Kunjumonthodu further northwards through stream branch and crosses the reservoir east of Idimuzhanganpara and goes northwards touching the northern boundary of the sanctuary at the elevation point of 780m at Thirthakaramala. The buffer zone consists of human inhabited areas viz. Rosemala and Kallar estates. The vegetation of this zone mainly consists of southern secondary moist mixed deciduous forests and west coast semi-evergreen forests. Since the pressure of human population within the sanctuary pose severe conservational problems, the efforts should be directed towards their effective rehabilitation.
The western most portion of the sanctuary beyond buffer zone constitutes the tourism zone. It consists of an area of 27.1 sq.km, which is about 16% of the total sanctuary area. The entry point to this sanctuary is in this zone. It consists of west coast semi-evergreen forest, reed brakes, southern secondary moist mixed deciduous forests and small patches of grasslands also. A portion of the reservoir is falling in this zone also. Only regulated tourism is permitted in this zone.