Vulcanologists say that Mt Ruapehu's crater lake has risen 15cm as a direct result of a small earthquake on Monday evening.
A new research project measuring the lake's temperature and level has allowed GNS scientists for the first time to correlate a small rise with a single tremor.
The magnitude 2 tremor did not cause an eruption or the water temperature to change.
But it did mean an extra 20 million litres, or eight Olympic-sized swimming pools of water, flowed into the lake from the hydrothermal system below, and it has continued to steadily overflow.
GNS hopes its latest data-gathering experiment will help identify potential hazards from the crater lake.
