15 February 2019
By generation fleet file I assume you mean the spreadsheet found here. If so, then we should point out that that file has been manually put together on a best endeavours basis by somebody that takes a passing interest in this stuff, i.e. it's not their day job. But more importantly, the information is not drawn from any market system where individual units are referenced by a 4-character code.
That said, the unit codes ought not be too hard to align with a plant. The first three characters are a site location, e.g. HLY for Huntly, and the number is the unit, e.g. HLY5 for Huntly unit 5, the large CCGT plant at Huntly. Where you're going to have some difficulty is when a station with many units is offered in by station; this is the norm at hydro stations. For example, the offer called 'WKM2201 WKM0' is for the Whakamaru hydro station. But the system operator doesn't care which unit the station owner uses to deliver the energy so the station owner just uses a zero to denote any or all of the units at the station.
Hope this helps. I would like to say that we should soon be in a position to publish a comprehensive generation plant list with lots of useful information by plant. But I can't. It's on our TODO list but never quite gets to the top of the list - maybe later this year!