Preheat oven to 190 degrees C. Grease and flour a 20x30cm cake pan.
In a large bowl, sift together the flour, sugar, baking powder, bicarb soda and salt. Make a well in the centre and add margarine, eggs, vanilla and bananas. Beat 2 minutes at medium speed with electric mixer. Pour batter into prepared pan.
Bake in the preheated oven for 25 to 30 minutes or until a toothpick inserted into the centre of the cake comes out clean. Allow to cool.