Lake Alexander is a small alpine lake approximately 400m long and 200m wide which feeds the Tummil River in Marlborough south of the Waihopai Valley in the Ferny Gair Conservation Area. The area is extremely rugged and unusually, for an area south of the Wairau River, heavily forested. Access to the Tummil is via a 5km 4WD track across private farmland and requires payment of a fee at Avonlea Lodge. Avonlea Lodge also provides guided hunting expeditions in the area. From the end of the farm track, a DOC tramping route follows the Tummil River to Lake Alexander. The route involves numerous river crossings and is steep in places. The route takes 3-4 hours to arrive at the lake. A six bunk hut at Lake Alexander provides accommodation. An abandononed historical hut is a few metres from the modern hut.
The largely undisturbed native bush in the area is home to a large number of indigenous plant and animal species.