Dont be fooled this little island hasn't been surrounded by water for 10 years or more. There any many trails to follow around this park, they cross and switch back all over the place and can be great to explore.
But if you want to make it easy follow the gravel path from the car park to the trail head, which is hidden behind the painting information board, take the trail to the left and you will eventually find your self on the “dark island” as I like to call it. The tress here arent like the surrounding bush, its darker and trunks of the tress rub against each other when its windy.
But you dont have to enter the dark trees to find the geocache, but once you have found it, go for a walk around the island. Maybe even take one of the paths off the dark island, through the dried out lake bed and into the yarra flats bush land.