She is remembered for leading the 1975 Māori land march from Te Hāpua to Wellington, a distance of 1,100 km (680 mi), at the age of 79.

After high school, she returned to Whakarapa (later Panguru) where her father wanted her to enter into an arranged marriage with Tureiti Te Heuheu Tukino V, leader of Ngāti Tūwharetoa.

This would have obstructed current use of the land by local iwi for gathering seafood and for racing horses during the drier months.

Heremia challenged the lease in court, and Cooper led a group of protesters to fill in Holland's drains.

The protesters were eventually charged with trespassing, but they had stalled events long enough for Heremia to be successful, and the Marine Department withdrew the lease.

[3] In 1916, she returned to work at the co-operative store, and around this time, met her first husband, Richard Gilbert, of Ngāti Wai.

There she played a leading role in business and the community, including in 1923 calling a hui that led to the name of Whakarapa being changed to Panguru.

[3] During September and October 1975, the nearly 80-year-old Cooper again became nationally recognised, walking at the head of the Māori land march from Te Hāpua to Wellington,[7] a distance of 1,100 km (680 mi).

[8] The slogan of the march was "not one more acre of Maori land"; they demanded acknowledgement of property rights under the Treaty of Waitangi.

[16] On 3 February 2020, a memorial at the Waipuna Marae was unveiled in Panguru, Hokianga, in the presence of prime minister Jacinda Ardern.