Please give me suggestions about how to improve this essay about South Sea Islanders in Australian history.The presence of South Sea Islanders has brought in their magnificent traditional habits,...

You might want to consider changing the thesis of this paragraph.  In the way that history is taught in the United States, at least, there is a great emphasis on what oppressed people do to resist oppression and to remain resilient in the face of harsh conditions.  You allude to this in your second sentence.  I would simply use the second sentence as my thesis.  I would change a few things to make it grammatical, though.  I would say “Despite the controls over their lives as they worked as indentured labourers (please note that you are no longer calling them slaves here)…”  You also say “lived” when you mean “lives.”

I would then add a second sentence that said something like “they maintained traditional habits, cultural beliefs, foods, and religions.” 

You mention foods that were eaten only on special occasions.  What did they eat on regular occasions? 

Finally, your last sentence does not have a clear purpose here.  I would add to it to make its purpose clearer.  I would say something like “Islanders also kept their spirits up by learning new ways of recreation.  In particular, they have excelled in the sport of rugby league.  In these ways, Islanders coped with the harsh conditions by keeping their old culture and by adopting some aspects of Australian culture.”