The correct answer is - Darwin.
The biggest city in Australia, located north of the 15°S latitude, is the city of Darwin.
Darwin is a city that is located on the far north of the country, on the shores of the Timor Sea. It is the most important city in Australia on the northern coast, and it is very important for the trade with the neighboring region of Southeast Asia.
The city itself is relatively isolated, and the next bigger cities are very far away, so it is a center of all sorts in its respective region.