Im a medical law student in the UK and just let me say, it's tough to prove what you want to prove. It really comes down to the facts of the case, but in the UK (and most commonwealth law is similar) they problems you will have is with the consent form that was signed and with proving legal causation...
I wont go into it, but basically, if the consent form signed warned of exactly this, then I'm really sorry but they have covered their back in that respect... you would contest the content of the consent for itself, but that's taking a onctractual angle to it and, well, there's poetntial but still, unlikely to succeed...
How ever if the consent for did not warn of such severe scarring (and im guessing it will be slightly up for interpretation) you will have to prove that the law should hold him accountable - legal causation. He was under a duty of care and did actually cause the "injury", but was there necessarily a breach? In uk we rely on a sace called "Bolitho" which resetablished the long running rule from "Bolam", stating that is a medical procedure is supported by a reasonable, responsible body of professional medical opinion then it does not constitute a breach. I am not sure how close this is to NZD, but under such test there is room to argue that even if he does find other docs that would have made a 2 inch incision and what ever stitching in your particular case, such practice could be considered a breach given that this is a plastic surgery case. The doc is being paid a very high amount of money, therefore the law holds them to a higher duty, and given that the surgery is cosmetic and aesthetic, a 2 inch pruple scar would certainly be against acceptable practice...
Another points that may be useful is regarding the information you were given, did you read the contract? Did he inform you of all the risks including scarring? It's not simply enough to shove a form under your fathers nose and say sign it. There are 2 people involved here, the patient (youself) and the consent given (your father), and cosmetic surgery is a pretty much undecided upon area in terms of consent issues... there is room for debate on who and what on he should have informed...
Additionally, even if the contract did claim to warm of scarring, you again could claim that your scarring is so severe that it shouldnt be within such scope because of it's being a breach of duty...
Anyway, I'm sorry i cant help you more but it really does come down to specific facts and im not really aware of NZD law...