Thanks for the comment.
Based on this information, it looks the that it is very serious problem if the hands on plumber do the job without supervision by a licensed plumber. But it is not clear for me that what does "supervision" means.
Foreign plumbers are required to have a valid work pass and licence, she said. Unlicensed plumbers carrying out critical plumbing works without supervision by a licensed plumber could be fined up to S$10,000 and jailed up to three years.
https://www.todayonline.com/singapore/stricter-licensing-regime-plumbers-kicks-april