But it's been a long campaign, and there are many years of it yet, DOC Tongariro technical adviser Harry Keys said.
Heather was introduced into the park by a warden between 1912 and 1921. He wanted it to feed grouse, and wanted grouse shooting as a sport.
The introduction was stopped by government, on the advice of scientists. But it was too late, because the heather had taken hold. By the 1960s it was out of control.
It continued to spread until the 1980s. The Conservation Department (DOC) held a first workshop about it in 1988, Dr Keys said.
European beetles that eat heather were suggested as a solution. In the early 1990s the beetles were put through a "starvation test", to make sure they wouldn't eat any other native plants. It took four years to be certain of this.
They were finally released in January 1996, in two places in the park, but they only survived in one of them.
In 1999 their work became visible, and beetles were collected and moved to other parts of the park.
They haven't bred up quickly, Dr Keys said, because they are small insects in a cold environment, and feeding on foliage with low nutrients. A larger, more robust species is being bred in Rotorua.
It will be introduced in two years, and DOC staff hope it will be able to survive and do its work at altitudes as high as heather can reach.
Dr Keys is not surprised it has taken 20 years for much sign of heather dieback, and he is heartened to see native plants re-emerge.
"We always knew this was an ecological scale process, not an annual business cycle process," he said.