Hong Kong immigration law is very clear about the need for foreign nationals to secure permission to work in Hong Kong.
If you intend to come here to take up employment, paid or unpaid, for long or short duration, for a local or an overseas employer, you need to secure the consent of the Director of Immigration.
This means that you have to make an application for an employment visa, before you arrive in Hong Kong to commence the employment.
The Hong Kong Immigration Department are very familiar with the need for foreigners requiring such short term temporary employment visa consents and will work earnestly with you to try to finalise such applications as quickly as possible, even anticipating round-pegs-in-square-holes quirks over such issues as a valid sponsor and the employing entity.
So, whilst most foreign nationals secure a 90 day or even a 180 day visitor visa upon arrival in Hong Kong, undertaking any work on such a visitor visa will be a breach of your conditions of stay and will make you, and other person party to such an employment, vulnerable to prosecution for a breach of conditions of stay.
There’s no need to be concerned about getting the employment visa so don’t be hesitant (or lazy) in applying.
Start the process at least 4 weeks before you’re due to arrive, fully disclose your intentions and objectives and let the HKID take a view and ensure that you’re compliant with Hong Kong law whilst you’re on the job here.