If you see lines on either screen of your Nintendo DSi during game play, then it is likely one or more of the components that create the image on the screen are damaged. This generally occurs from an external event, e.g. the device being placed under stress or subjected to impact.
Such external influences are the result of improper handling of devices, accessories or games. In this case, the warranty becomes void and repair charges will apply.
To correct this issue the Nintendo DSi will need to be returned to Nintendo Australia for inspection/repair.
Important Note: If you are seeing lines on the screen that slowly fade away after turning off the system, this is normal.