Hi Cricfor,
For your case since I guess your qualifications are not related to Engineer, and your working experience is not related to "Engineer". In this sense, I believe you cannot register as an Engineer in IPENZ, and you cannot get any reference letter get from IPENZ.
For my understanding, I believe you can still claim points for your experience. The most important are you have to stage clear in your EOI/ITA, and can prove that your experience of "Software Engineer" is relevant to software development, and the job duties can match with ANZSCO Software developer / Programmer.
What you can do is to provide a reference letter from your previous employer's manager to stage your job duties.