Litchfield National Park can be visited all year round.
The main road is all bitumen. But the four wheel drive tracks will usually be closed during the wet season (Nov – April), so some areas will be inaccessible (The Lost City, Tjaynera Falls, Surprise Creek Falls).
But the main attractions, like Wangi and Florence Falls, can be visited all year. The wet season is actually a good time to see them. The falls will be thundering, not just dribbling, and there are fewer tourists around. Litchfield National Park gets very busy during the peak of the dry season (June-August).