Now, you are back to saying that this is an essay about how the islanders have impacted Australia. But you really have said very little in this essay about how this has been true. This paragraph, too, does little to prove that the islanders have had an impact. At least half of this paragraph (the first five sentences) shows nothing about how islanders have impacted Australia. Your final three sentences in the first paragraph of this excerpt claim that islanders have had major impacts, but give no proof.
Overall, then, this essay does not really prove what you have set out to prove. You say that the essay is about the impact islanders have had on Australia, but your essay is really about what a hard time they had and how badly they were treated. If you are allowed to do so, you should change the whole topic of the essay. You should change your introduction and conclusion to say that this is an essay about what a hard time these islanders had. If you are not going to change your thesis, you need a lot more information about how islanders have had an impact on the country.