In the year ended March 31, 2016, KASM generated about $25,500 in income, of which $17,200 came from membership and donations.
It spent $21,300, with its biggest expenses legal costs at $8,800 and wages at $8,600.
KASM argued it was only opposed to non-essential seabed mining, not all seabed mining, and its advocacy work was ancillary to its main purpose of advancing education, but the board was unconvinced.
The group also argued the relevance of a Supreme Court finding on Greenpeace's charitable status, where the majority of the bench found that political advocacy could be charitable, depending on the goal and its promotion, particularly where environmental objectives were at stake.
While the board considered KASM's stated end goal of environmental protection, and the way it carries out activity, to be charitable, it said it couldn't determine whether the group's focus on stopping seabed mining was a charitable public benefit.
"The board considers on balance that the wider consequences in prohibiting and opposing seabed mining are too varied to assess the society's advocacy against seabed mining as clearly advancing a charitable public benefit," it said, adding that it considers "any public benefit is unlikely to be capable of demonstration by evidence".
KASM also argued its advocacy was ancillary to its educational work, but the board said its educational activities have been aimed at advocating the prevention of seabed mining.