By december, Nzm station would be accessible by Metro also.
Considering you will have luggage so i wouldn't suggest you to interchange 2/3 metro's to reach NZM as that would be hassle, so it will be better you take an Ola/Uber from Airport ask him to drop you at Durgabai Deshmukh South Campus Metro station (GMaps link) from there the metro will directly drop you at NZM stn. within 25 mins.
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more... Your flight arrives by 5pm, and you come out by 5:30 max, cab will take max 25 mins to drop you at DD South campus, 10 mins for catching metro and another 25 mins till NZM and then another 10 mins from NZM metro stn to Railway station.
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5:30pm + 25 +10 + 25 + 10 - By 6:30pm you should be at Nzm stn. and still have 15 mins of buffer.
Also as per 12903's historic Data, 25% of the times it arrives at NZM with 15 to 60 mins delay plus it will be December month when trains time tables take a back seat in North, so quite safe bet.
Go ahead.