This pumice raft (see right) is the product of a submarine volcanic eruption from one of the multiple of seamounts that are part of Kermadec arc north of New Zealand. Exactly which volcano is the source of the pumice is unclear — early on, it was suggested that Monowai was the source, but that seems to be in some question based on the location of the 26,000 square kilometer pumice raft (see below).
The raft is located to the northeast of Raoul Island, one of the active volcanoes of the Kermadec Islands that is above the sea surface. The next known active volcano to the northeast of Raoul is Monowai, thus the suspicion that it may be the source. Likely the only way this actual source will be identified is through matching the composition of this pumice with that of known material from Monowai (or another Kermadec volcano).
Rafts like this can travel great distances – some of the pumice rafts from the Krakatau eruption in 1886 washed up on African beaches months later — so trying to determine the exact source of the pumice when it is so widespread is challenging. However, this is likely a significant eruption based on the size of the pumice raft across the ocean surface. Monowai does seem reasonable, though, as it has produced significant eruptions in the last few years.