If you can't get an answer by using a SearchCursor (as per @StephenLead 's answer above) you could look at downloading the pyproj library. As long as you know the relevant definition of your projection (which you can get from www.spatialreference.org), you can easily convert from one projection to another. For example:
import numpy as np
import pyproj
lonlat = np.array[[135.5, 135.4], [-47.5, -47.6]] #replace with your own data
vicgrid94 = pyproj.Proj(init='epsg:3111') #replace this with your own projection
gda94 = pyproj.Proj(init='epsg:4283') #Australian geographic coordinate system
#WGS84 = epsg:4326 for comparison
#first option
xy = pyproj.transform(gda94, vicgrid94, lonlat[0], lonlat[1])
#first option
print vicgrid94(xy[0], xy[1], inverse=True) #With inverse=True transforms coordinates
#in projection to lat long
#Note: floating point error will be obvious