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I would like to know if this is fair. I had an ecu replaced and a service and a lot of things repaired with my 1992 Mitsubishi Lancer last July 2012 at a mechanic in Petone. Last week my car broke down again and seemed like it was a similar problem. I took it in to the same place and they told me on Thursday the 31st of January that they will look into it and are looking at some wires from the ecu to the fuel pump or something along those lines. Didnt hear from them until today when they told me it was all done and my car was ready for pick up and it will cost $531. I don't even know what they did, they didn't tell me before the job was going to go ahead. I feel very ripped off especially because I paid about $1200 in July to have that all fixed.

If the replacement ECU repair has lasted this long and if the repairer has said the problem is related to the fuel pump wiring, it's unlikely that the recent failure is the same thing as the first, so on that basis, it doesn't sound like you have been ripped off.
However, there are a couple of question marks. Firstly, the repairer should have got your go-ahead to carry out the repair, although he would have had no idea what he was going to find before going ahead with the diagnosis. I guess the assumption was that because you had taken the car to him, that you wanted him to fix it - perhaps a reasonable assumption for him to make, but maybe you and he should have agreed a maximum dollar amount/time spent on diagnosis before he might call a stop to the job and keep you further informed.
In summary, while it seems that there may have been a miscommunication, electrical diagnosis can take considerable time. Very often, it can take several hours to trace the problem and minutes to fix once the problem is found.
Without knowing exactly what was done, it's difficult for us to say with certainty, but it doesn't sound like you have been ripped off, just maybe a breakdown in communication.

Well i dont think its fair that they just went ahead and did the job without me knowing about anything and without any communication whatsoever

We'd agree with that. Lack of communication is the most common cause of disputes with repairers.