You don't always need an undergraduate degree to study postgraduate
While 'postgraduate' implies that you need to have already graduated from something, that's not always the case. Many of our postgrad courses (mainly graduate diplomas and graduate certificates) are available to people with no formal qualifications, provided they have significant work experience in their chosen field.
So if you plan to study a postgraduate course in the same field as that in which you're employed, view the relevant course page and check the 'admission criteria'. You may be eligible to enrol solely on the basis of your work experience.