Sydney Harbour is a harbour located near Sydney, Australia. This is where Marlin and Dory go to find Nemo. Sydney Harbour was first seen (as a word) on the mask. Some of the inhabitants of the harbor include pelicans like Nigel, Errol and Gerald, various seagulls, and crabs.
When Marlin and Dory arrive at the harbor after being shot out of the Whale's blowhole, they are suddenly swallowed whole by Gerald, who then threatens to eat them only to stopped by Nigel, another pelican who knows who Nemo is and his current whereabouts since he is constantly visiting Dr. Sherman's dentist office. When Nigel notices a flock of seagulls arriving to eat Marlin and Dory, he scoops them up and flies them to the dentist's office while luring the gulls into a sailboat, causing them to crash into the sails while the pelican flies through the empty space formed by the boat's mast.
When Marlin thinks that Dr. Sherman's niece Darla had killed Nemo by shaking his plastic bag (he's pretending to play dead to trick Dr. Sherman to flush him down the toilet), Nigel returns him and Dory to the harbor in grief, and the two to part ways, only for Nemo to emerge from a sewage pipe leading from the dentist's office and meet Dory, who then asks him to find Marlin. During their search, Dory is accidentally caught in a fishing net, causing Nemo to warn his father and prompting Dory and the other fish to escape the net by literally swimming down, causing the net to be weighed to the point that it literally gets torn apart, allowing the fish to swim free. Nemo's plan succeeds, and the three all return home together.