well the only problem i can forsee will be a weaker tasting beer, dont throw it out. It has obvioulsy started fermenting and you wouldnt expect the fermentation time to take any longer because you still used the same amount of water.
You would still add the same amount of suger as a regular beer, ie a full teaspoon for a tallie or a half teaspoon for a stubbie, or a 2 carbonation drops per tallie and 1 for a stubbie.
You could still take a hydrometer to test when it is ready, you woulc be looking for about 1005 (or the same reading two days in a row), although the alcohol content shall remain a mystery to us all, you will prob need excessive testing to determine the