Hi @mia-clubfido-co-
Similar question from about six months ago - link
I still hate giving 'it depends' as an answer but... it depends!
In this case, it depends how your new business is set up (are you working as a Sole Trader - i.e. self-employed - or as a limited company?).
In practice, £600/month is below the current income tax & NI thresholds - so there is no tax/NI to be paid. However, the reporting (to HMRC) of the payment may be important because when you say 'earning', what matters is how the money is being paid/taken.