One reason could be to study rare magical plants. Australia has a diverse range of flora, and some of it might have magical properties that are not found anywhere else. So, he travels there to learn about these plants and their potential uses in potions or other magical practices.
It might be related to Hermione's research. She may have read about some unique magical phenomenon in Australia that she wants to study first - hand. And Harry, being her loyal friend, goes along with her. For example, there could be a special type of plant with healing properties that only grows in certain parts of Australia.
Perhaps he was on a quest for a rare magical ingredient only found in PT.
Harry could be following a hunch. He might have found some evidence that links Snape to a solution he needs. For example, he could have discovered a note written by his father or mother that mentions Snape's ability to deal with a certain type of dark magic that is now threatening Harry. So, he takes a chance and goes to Snape, hoping against hope that Snape will be willing to help.
In this fanfiction, Hermione would be greatly influenced. She would be eager to explore the Australian magical libraries and discover new spells and potions. Ron might gain a love for Australian food, which could change his physique a bit. The kids would be more open - minded, as they are exposed to a new and diverse magical culture. They might also become more independent, as the Australian wizarding community could have a different parenting style.
The unique flora and fauna of Australia can play a big role in these fanfictions. There could be magical versions of kangaroos or koalas. And the way magic interacts with the Australian environment is also different. For example, spells might be affected by the extreme heat or unique geological features of Australia. This makes the fanfictions set in Australia stand out from the more common European - based Harry Potter stories.
They might go into hiding because there are still some remnants of Voldemort's followers who are seeking revenge on Harry. These followers could be more dangerous than before as they are desperate and hiding in the shadows. Ginny, being Harry's wife, is also in danger, so they choose to go into hiding to avoid any potential attacks.
They might go to the Burrow to reconcile. Fanfiction often explores the idea of enemies becoming friends. The Burrow, being a place full of warmth and family, could be a good setting for them to start over. It's a place where they can put aside their differences and get to know each other better without the pressure of Hogwarts politics.
They could be on a mission to exchange magical knowledge. Hogwarts has its own set of spells and magical techniques, and so does the American wizarding world. They might go to America to learn new things and share their knowledge as well.
One reason is that it allows for a lot of creativity. Writers can explore different aspects of the wizarding world that haven't been fully covered in the original books. For example, they can create new magical traditions or spells based on Australian Aboriginal culture. Also, the idea of Harry and Hermione in a far - away and exotic place like Australia is appealing to fans who want to see their favorite characters in new situations.
In some fanfictions, they might go there to search for something. Maybe they are looking for a magical artifact hidden in Australia. It could be a powerful charm or a lost spellbook that could help them in their fight against the Dark forces.