A government minister, Ralph Regenvanu, said the decision was made by cabinet this morning.
The volcano at the island's centre, which has been erupting in fits and bursts since September, has increased its activity in recent days.
On Thursday, reports from the island indicated that ash fall was so severe the sun was blocked.
Hundreds of Ambaeans have already opted to leave voluntarily, most of them to Vanuatu's large, north island of Espiritu Santo.