Preheat oven to 180C/160C fan forced. Grease a 20cm (base size) round cake pan and line the base with baking paper.
Place lemon syrup cake mix (keep the lemon syrup mix aside), butter, eggs, milk and lemon rind in a bowl. Use electric beaters on low speed to beat until combined. Increase speed to medium. Beat, scraping down side of bowl with a spatula, for 2 minutes.
Spoon into prepared pan. Bake for 40-45 minutes or until a skewer inserted into the centre comes out clean.
Meanwhile, place the lemon syrup mix, tonic water and lemon juice in a small saucepan. Bring to the boil over medium heat.
Reduce heat to low and simmer for 8 minutes or until reduced to about 160ml (2/3 cup). Set aside to cool slightly. Stir in Australian Garden Gin.
Place cream, mascarpone and 2 tablespoons of cooled lemon syrup in a bowl. Use electric beaters to beat until firm peaks form.
Cut the cake vertically into ten 1.5cm thick slices. Line the base of a 7cm-deep, 20.5 x 31cm dish with half the cake slices. Brush with one-third of the syrup. Top with half the cream mixture.
Top with the remaining cake slices. Brush with half of the remaining syrup. Top with remaining cream mixture. Place in the fridge for 2 hours or until chilled.
Sprinkle with lemon rind. Drizzle with the remaining syrup.