No, it’s not mandatory that you hold a Welder qualification for the Welder skills assessment to migrate to Australia.
You are required to either hold your completed apprenticeship papers as a Welder (Please note these qualifications could fall under many engineering qualifications) and 3 years of paid work experience pre or post qualification for the Welder skills assessment.
If you don’t have an NVQ3, you are required to have a minimum of 5 years paid work experience as a Welder. 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 for Australian migration.