Preheat oven to 160⁰C fan forced. Grease an 18cm x 28cm rectangular slice pan and line base and sides with baking paper, extending paper 2cm above edge of pan.
Place Arnott’s Lemon Crisp biscuits into the bowl of a food processor and process until the mixture resembles fine crumbs. Add butter, process until just combined. Press the mixture over the base of the prepared pan. Chill for 30 min.
In the meantime, place condensed milk and lemon juice in a bowl and stir until thickened. Pour mixture over the base and bake for 20-25 min until set. Cool, then chill 1½ hrs until firm. Cut into squares. Dust with icing sugar and garnish with lemon zest.