Welcome to the forum.
Are you only having 1200 kilojoules? Perhaps you meant 1200 Calories. 1200 Calories is the recommended minimum energy intake for healthy adult women attempting to lose weight on a calorie-restriction diet. It's a figure you'll see quoted on many weight-loss sites. That's about 5020 kilojoules. According to some medical authorities an average body can sustain an intake of around 25 per cent less than the average energy requirement. YMMV.
But, if you DID mean 1200 kilojoules then that's going to result in some problems: Starvation mode arises because the body basically prefers to hold onto its energy stores and would rather reduce its metabolic rate than let go of fat. A reduced metabolic rate will mean you need less food, will lose less weight, and if it stays in a reduced state after your diet ends then it will be far easier to put weight back on.
How does it reduce its metabolic rate? It reduces the amount of energy it needs - by slowing or shutting down basic life and health sustaining processes such as cell repair, growth, nutrition, chemical conversion, immune system, organ function, bone maintenance, hormone production and so on. The body also clings to its valuable fat stores by regularly taking energy from muscle and internal organs (don't forget the heart is a muscle).
However, the above is not medical advice, it's just a distillation of commonly available knowledge. I do, however, suggest you to speak to your GP about your diet as he may be able to recommend some modifications to make it safer and sustainable.
Jeez, I sound like a great Welcome Wagon, don't I!