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.
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.
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.
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.
One popular 'Harry and Hermione Australia fanfiction' might be those that explore their adventures in the Australian wizarding community. Maybe they go there to study some unique magical creatures or spells that are only found in Australia. Another could be about them solving a mystery that has its roots in Australia's magical history.
One reason could be to search for a particular magical item. Maybe Dumbledore left behind a clue that led them to believe there was something important in one of the vaults at Gringotts.
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.
It could be for safety reasons. If there's a threat at Hogwarts or in the wizarding world in general, Hermione's house might be a safe haven. Her family might have some protective spells around the house that Harry can take advantage of. Also, Hermione's parents are Muggles, so it might be a place where the dark forces wouldn't expect them to hide.
Sometimes, even antagonists from Hermione's past in the UK might make an appearance in the story. Maybe a Death Eater who escaped justice and is now also hiding in Australia. This would create a tense situation where Hermione has to deal with this old threat while also trying to build a new life in a foreign magical land.
One possible plotline could be Hermione starting a new life in Australia, perhaps opening a magical library or study center. This would allow her to share her knowledge with the local wizarding community.