Nobody is allowed on Whakaari.
The advice from the Bay of Plenty regional Council Harbourmaster is for boaties to avoid the waters around the island.
If the island were to erupt, the main potential effect on people and property on the mainland would be ashfall. There has been no ashfall in recent eruptions because they were too small and it depends on things like wind direction.
Whakaari is 48km from the coastline. It is rare for ash to travel more than a few kilometres from a volcano during an eruption.
If ashfall DID reach the mainland, it would not be toxic to people or animals. However ashfall could irritate skin, eyes and lungs and be corrosive to some surfaces.
For more information:
https://www.geonet.org.nz/volcano/whiteisland