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MEL-AKL is typically not very full but it depends on season (summer is peak), day of week (business vs leisure travel), whether there is a sporting or other event on in MEL or AKL, any special promos, etc.

I doubt you'll be allowed to keep your seat, but you could get lucky.

Australian law requires all pax to disembark in Melbourne, and you'll need to go through security (but not immigration or customs on the way over) even in transit. As pax in J/F you will have access to Cathay Pacific's lounge which is okay but not too flash (QF and NZ are much better).

Actually your fare doesn't seem too low to me - I've seen much lower on EK.