Hi Christian,
While we cannot provide the exact answer, you can make reasonable assumptions. Considering the limited options for consumer goods (they only have 1 grocery store on the island), there is a predominance of single-use plastics in such contexts. So, plastics bags, food packaging, and beverage containers might contribute to it.
But if you look at the photos and interactives of the fenced waste area, you can see what other waste is on the island.
This report from Queensland, gives you a make up of waste in the state, from there you can deduce it for Saibai by taking into account the population, as well as the facilities available. Hope that helps.
Cheers,
Sai
Sai Rupa Dev