Date of birth change on passport

Hi, I need help in correcting my date of birth on my passport. I got my birth certificate recently and as per my birth certificate my date of birth is 11/08/1988, but on passport my date of birth is 20/05/1988. I got my first passport issued on 2001 when I was minor, and after that I got my passport renewed several times. Now I am working in New Zealand and for the purpose of Residency application I needed my Birth Certificate. So I got my birth certificate last year and as per the date of birth on my birth certificate I changed my SSL Certificate and my marriage certificate. But when it came to change the date of birth on my passport it took me to the real trouble. I need a court order from the Honorable High Court of Kerala so that I can change my Date of birth on my passport through the High Commission here in New Zealand. So I filed a case for that through a lawyer. Now the opposition lawyer acting on behalf of the Passport Office says I have already provided a Birth Certificate from the same authority when I first applied for passport back in 2001. Me and my whole family are shocked hearing that news, because when I first applied for the passport back in 2001 I was minor and my Parents were abroad; it is my late grandfather who did all the application and all for me and for brother as well. So none of us are aware if he has provided any birth certificate documents at that time. The lawyer now says that it is not safe to move further forward with the case. He says there is no other option than withdrawing the case from the court. I am completely out of mind and don't know what to do. if I cant find a solution for this I have to leave New Zealand by leaving all my dreams behind here. Somebody please provide a solution. Thanks