Eruptive activity increased early Tuesday but there is no immediate threat to homes or infrastructure, the observatory said.
Volcanic gas known as "vog" may reach downwind subdivisions near the park, the observatory warned.
The park encompasses the summits of two of the world’s most active volcanoes: Kilauea and Mauna Loa. But the eruption's location in a remote wilderness area more than 6 miles (10 kilometers) from the nearest road doesn't make public viewing possible, according to park officials.
During a Tuesday morning helicopter overflight, geologists said they observed fountaining eruptive fissures and active lava flows on the floor of Napau Crater.