Visiting the park is recommended between late December and late February, during the southern summer. Not only is the weather more hospitable, but daylight hours are very long given the extreme southern latitude. Outside of this time frame, the weather becomes too extreme for the majority of the public, and daylight dwindles to only a few hours a day.
Don't expect world class water park and you'll have fun. There are slides, a roller coaster, and a lot of water. Entrance fee isn't high, bathrooms have showers, you can relax on a long chair on a grass and sunbathe if you're tired of water.