looping and searching in numpy array

[Heli]

Dear all, 

I need to loop over a numpy array and then do the following search. The following is taking almost 60(s) for an array (npArray1 and npArray2 in the example below) with around 300K values. 


for id in np.nditer(npArray1):
                  
       newId=(np.where(npArray2==id))[0][0]


Is there anyway I can make the above faster? I need to run the script above on much bigger arrays (50M). Please note that my two numpy arrays in the lines above, npArray1 and npArray2  are not necessarily the same size, but they are both 1d. 


Thanks a lot for your help,