Vanuatu's Geohazards Department says the latest activity on Ambae is a reminder that island's volcano remains unstable.
The department's Esline Garaebiti said Monday's eruption was part of the third phase of activity which began on June 21.
Since September the activity at the Manaro volcano has been uneven, sparking evacuations on two occasions to nearby islands.
Ms Garaebiti said the alert level remained on two with a low to moderate chance of increased intensity.
However she said because the latest eruption occurred when there was not much wind, a plume of ash and smoke was pushed around 6,000 metres into the sky.
She said locals and those on surrounding islands could be impacted from the ash over the coming days.
"It's unpredictable," she said.
"All we know is that the system is unstable and at anytime we will expect this type of eruption and according to the recording that we have, the assessment data we are recording on that island, the activity is still ongoing."