Hi Thomas,
I was driving my partner's car on one occassion and received a low level speeding infringement in NSW. My partner received the notice and we both had to sign off that I in fact had been driving his car at the time. So I am not sure how your company was able nominate you as the driver without your consent or knowledge if this is in fact the case.
Also, who sent you the letter regarding the fine? From your description it doesn't sound like something that the NSW Department of Roads and Maritime Services (RMS) would send to you. What happens when fines are transferred in NSW from one person's name to another is that a new infringement notice is issued directly from RMS and then a follow up notice if the first one wasn't paid and so on.
Can you please give some more information about your situation, specifically whether the RMS has issued you the fine directly? I also agree with Paul that you should get in contact with the Department directly as well.
I hope you're able to resolve this soon.