It is not mandatory to have a qualification for the Fitter skills assessment for Australian migration. You are required to either hold your completed apprenticeship papers as a Fitter. Please note these qualifications could fall under many engineering qualifications (mechanical engineering for example) and 3 years of paid work experience pre or post qualification.
So, if you are from the UK and hold an NVQ3 as a Fitter, you would only require the 3 years.
If you have NO NVQ3 for example you are required to have a minimum of 5 years paid work experience as a Fitter. It is common for clients to be assessed without holding any formal training or qualification so we use your 5 years of paid work experience as proof.