Complex sentence: It is sometimes alleged that their employment in Australia was a form of slavery, due to the belief that some Kanakas were recruited by kidnapping (or "blackbirding" as the practice was known at the time).
Split into simpler sentences: It is sometimes alleged that their employment in Australia was a form of slavery. This is because some Kanakas were recruited by kidnapping (or "blackbirding" as the practice was known at the time).