dd bs=1k if=image.nrg of=image.iso skip=300

Essentially what this command is saying is that using 1k block sizes, from the input file of image.nrg (our Nero image that needs converting), create an output of image.iso (the desired ISO image file), after skipping the first 300k of the input file (aka the Nero 300k header).

http://osxdaily.com/2009/09/25/convert-a-nero-image-file-to-iso/