Rotomairewhenua, or Blue Lake, is a small alpine lake high in Nelson Lakes National Park on New Zealand’s South Island. In 2011 scientists measured underwater visibility of 70 to 80 metres here, making it the clearest natural freshwater ever recorded and about as clear as distilled water.

Clear water and mountain reflections at Rotomairewhenua / Blue Lake, Nelson Lakes

Rotomairewhenua / Blue Lake at a glance

  • Location: Nelson Lakes National Park, in the northern Southern Alps, Tasman region.
  • Elevation: about 1,180–1,200 metres, among peaks close to 2,000 metres.
  • Size: just 2.45 hectares.
  • Name: Rotomairewhenua means “the lake of peaceful lands”.
  • Outflow: the West Branch of the Sabine River, part of the Buller River system.
Blue Lake’s transparent shallows beside the forest edge

Why Blue Lake is the clearest lake in the world

The record comes from a study by New Zealand’s National Institute of Water and Atmospheric Research (NIWA). Hydrologist Robert Merrilees measured visibility of up to 80 metres, beating even the famous Te Waikoropupū Springs, at 63 metres.

The secret lies upstream. Blue Lake is fed by springs that carry water from Rotopōhueroa / Lake Constance, which sits higher in the valley behind a dam of old landslide debris. The water seeps through that rubble, which filters out almost every particle. Water also passes through Blue Lake quickly, staying only about 24 hours before flowing on into the river, so sediment has no time to build up and cloud it.

Rotomairewhenua lying within a steep green mountain valley
Visitor standing on the gravel shore of Rotomairewhenua

A sacred lake for Ngāti Apa ki te Rā Tō

Rotomairewhenua and Rotopōhueroa are tapu, or sacred, to Māori, particularly Ngāti Apa ki te Rā Tō. Blue Lake was traditionally used in ceremonies for the bones of the dead. Maps now show both lakes by their original names alongside the English ones.

Keeping the water untouched protects its mauri, or life force, and its purity. The Department of Conservation asks visitors not to touch the lake at all: no swimming, no washing and no filling water bottles. A signposted stream about five minutes from the hut offers a cold dip instead.

Underwater photograph showing the clear blue water and lakebed at Rotomairewhenua
Green light and reflections beneath the surface of Blue Lake

How to get to Blue Lake

There is no road to Rotomairewhenua. Reaching it takes about two days of tramping through valleys and over high passes, and most people visit as a side trip from the Travers–Sabine Circuit. Marked routes continue south over Waiau Pass and west via Moss Pass.

Blue Lake Hut, a 16-bunk Department of Conservation hut, sits by the shore and welcomes more than 700 trampers a year. Tiny riflemen are common in the bush around the lake. Plan the trip as a proper backcountry tramp, with DOC’s current track and hut information, and allow extra days for weather.

Light patterns on stones beneath clear lake water
View down into blue water beside Rotomairewhenua’s pale shoreline

Protecting the lakes from didymo and lindavia

Two invasive microscopic algae, didymo and lindavia, are already present in Lakes Rotoiti, Rotoroa and Tennyson, and once established they cannot be removed. To keep Blue Lake clean, DOC is installing cleaning stations at Coldwater, Lakehead and Sabine huts. Clean any gear that is wet from rivers or lakes using the Check, Clean, Dry method, including a 10-minute soak in detergent solution.

For more of New Zealand, see the Nugget Point/Tokata guide, or read our hiking article before setting out.

Clouds and mountains reflected across Rotomairewhenua’s clear water
Outstretched hand above clear water with stones visible below

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