GNS Science has issued warnings over both Mt Ruapehu and White Island.
The volcanic alert level for White Island has been raised following the discovery of an unusual lava dome in a crater formed by the last series of eruptions in August.
Ruapehu remains at a heightened level of unrest and an eruption is more likely than it normally would be.
GNS Science head volcanologist Gill Jolly says lava dome growth on White Island can be accompanied by explosive eruptions, which could pose a danger to people on the island.
The alert level has been raised from one to two out of a possible five, and the aviation code to orange, indicating minor eruptive activity with no or minor ash emission.
Dr Jolly says scientists are monitoring the dome and if it's not growing then they will be less concerned.
The dome is about 20m to 30m across with spines sticking out of it.
It's been formed by fresh magma rising and pushing already cooled magma upwards.
On Mt Ruapehu, Dr Jolly says temperatures beneath the crater lake are higher than normal.
She says this indicates the volcano is partially sealed a few hundred metres beneath the lake and that might be causing a pressure buildup behind it.
She says the crater lake is quiet and its temperature has remained relatively low since March.