The national park of Kakadu is 20K squared kilometers, the size of Israel, has 8 different habitats and only 2 seasons: wet and dry. (Indigenous locals have 6 though so it evens out) For its natural importance it has been named a UNESCO world heritage site. Though it is a double listed heritage site as it also has great cultural significance for Australia as it is Aboriginal land.
Billabongs are inviting swimming places around the national park. They are peaceful looking crystal clear ponds left behind as rivers changed course. For the longest part of the year, billabongs are dry but fill up with the seasonal rain. However, full of fresh and salt water crocodiles, all rivers, billabongs, water holes in Kakadu are strictly no swimming areas. Or swim at your own risk.
Both species of crocodiles were once hunted until almost extinction in around the 1960’s but are now protected species and their populations recovering. So whilst it is tempting, I’d rather stay out of becoming a crocs meal.