Ok, so at first glance, I think that it isn't too bad, but you were correct in thinking you were retelling a bit.
Through Lieutenant Gardiner’s character, Grenville gives the impression that internal turmoil is exacerbated by external conflict. A short while after the arrival of the First Fleet, Gardiner joins an expedition where external conflicts between the natives between and the settlers is made apparent when the settlers lure and capture two Aboriginal men. Due to his contribution to the capturing, Gardiner realises that despite being “savages… their feelings are no different to ours”, thus causing his to be internally distraught by the violence he took part in. His internal conflict and his disapproval of the Fleet’s actions, is made known to his superiors, thus resulting in him being sent elsewhere to “begin a new colony where the ground is more fertile” {i cant remember if the quote exactly says this}. Rooke believes that this is an order of punishment and exile for Gardiner’s thought on capturing the natives suggesting that Grenville created external conflict to amplify characters inner distress. Gardiner’s involvement in physical hostilities contributed to his inner turmoil, thus making him exposed to negative repercussions.
My rewritten one...
Through Lieutenant Gardiner’s character, Grenville gives the impression that internal turmoil is exacerbated by external conflict. The Governor is the principal instigator of the external conflict, due to his "need to communicate" to the natives driven by the fear of annihilation. Although "He [the Governor] should not have given the order" to capture Wurragin and Boinbar, Gardiner is distraught, loudly berating himself for his part in "the hardest duty [he] was ever asked to complete". Due to his efforts in the kidnapping, Gardiner realises that despite being "savages... their feelings are no different from ours", clearly exemplifying the internal turmoil that is being enacted in response to the violence he was part of. Gardiner's subsequent posting to Norfolk island is seen by Rooke as a punishment, almost "exile", suggesting that Grenville created external conflict within the novel to amplify characters inner distress. Gardiner’s involvement in physical hostilities contributed to his inner turmoil, thus making him exposed to negative repercussions.
EDIT
Sorry, I just realised that this post was really useless, giving no actual feedback. So here we go.
To answer your question... I would prolly rely a lot more on well positioned quotes to tell the story, as otherwise you will be marked down. I hope my example shows that...
I would try for a touch longer, but under timed conditions it could be unrealistic, so yeah, just do the best ya can.
And overall feedback:
Try to be more familiar and confident with the quotes, vocabulary was good.
Length is alright, but greater development of ideas would be great.
Like, a stronger linking sentence back to your topic sentence would be perfect. :D