There is no formal qualification requirement for technical writers. In fact, in Australia, there is no "technical writing" degree, although one institution does offer a Science Communication bachelors degree.
Most undergraduates who are interested in a technical writing career study BA Communication and take majors in professional writing or journalism. Some institutions offer elective subjects in technical writing.
Many people become technical writers without ever planning to do so. Often people working in financial or industrial companies find that they have to write procedures or reports, and that they are good at it. Usually their colleagues are more than willing to let them take care of those duties, sometimes leading to a new career.
People who come to technical writing as a second profession often specialise in areas related to their original career. For example, a laboratory assistant who decides that she likes writing procedures and reports would be highly regarded as a specialist technical writer in pharmaceutical or cosmetics companies. Similarly, ten years of military service could lead to a career as a highly sought-after consultant to assist with the production of military tenders or requirements documents.
The disadvantage of specialising is that it limits your employment options. You may become identified as an "engineering writer" or "financial writer". Communication graduates rarely specialise to the same extent as technical writers with a "previous career". They are more likely to be placed in generalist positions and interact more often with subject matter experts. When demand for technical writers is low, recruitment agencies often demand multiple industry-specific skills.
The most important quality for a technical writer is to be able to present available information clearly. As well as skill with the written word, this requires the ability to collect, refine and organise information. While tertiary study can certainly make you better at these things, it cannot give you aptitude. If you don't like explaining "how things work", you won't ever be happy working as a technical writer.
Curiosity is also an important quality of good technical writers. If you don't know how things work, how can you explain it to others? The best technical writers are inveterate tinkerers who want to know "what happens if I push this button".
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