Dear Logesh,
Pl. go through the G.O fixing or revising the rates of basic wages in respect of any scheduled employment carefully and you'll find an explanatory note containing the method of calculation of D.A in tune with the hike in the cost of living index for every year till the currency of the concerned G.O. Therefore, no need for any separate authority or special authorisation, if any to calculate the D.A from time to time. But, generally there will be a circular from the Commissioner of Labour, Chennai every year regarding the rates of D.A payable with effect from April 1 of the current year calculated on the basis of the average hike in the 12 calender months of the previous year in respect of all the scheduled employments. You can also calculate it yourself.