The islands were also home to endangered New Zealand dotterels, oystercatchers, tui and kingfishers.
''So they're pretty important little islands for wildlife.''
Windust said he had been heartened by Northlanders' response to the theft, which included donations to cover the cost of replacements. A Haruru Falls family had also donated four new re-setting traps.
The group is planning measures to ensure any new traps can't be stolen.
Windust urged whoever took the Goodnature A24 traps to get in touch so he could understand their motivation.
There was no need to steal them because the group provided traps for people in the Paihia area who wanted to help reduce pest numbers.
''We won't judge you, and we definitely won't name and shame you. Maybe you're an animal activist, maybe you needed the traps elsewhere, we just want to know so we can help,'' Windust said.
Rats were easily able to swim back and forth between the islands, where there was no fresh water, and the shore. However, it was a mystery how the possum got there because the Australian invaders are not known for their swimming ability.