Preheat the oven to 350°F (175°C). Prepare a 9×13-inch baking pan by greasing, flouring, or lining it with parchment paper.

Grate the zucchini and squeeze out any excess moisture.

Whisk the yellow cake mix, instant vanilla pudding mix, and all-purpose flour in a mixing bowl until well combined.

Add the water, milk, eggs, and lemon zest to the dry ingredients. Mix gently until just combined; some lumps are fine.

Fold in the grated zucchini delicately.

Pour the batter into the prepared pan and spread it out evenly. Bake for 30-35 minutes or until a toothpick inserted comes out clean.

Cool the cake in the pan for 10 minutes before transferring to a wire rack to cool completely.