“I’ll Leave You to It” Means “I’ll Let You Continue”

If a colleague is busy and you say, “I’ll leave you to it”, you normally do not mean that you are abandoning them. You mean, “I’ll let you keep working, and I won’t interrupt.” Here, “to it” points to the task already in progress. The phrase often shows consideration and can also suggest confidence that the other person can manage independently.

For example: “Do you need a hand?” “No, I’m nearly done.” “All right, I’ll leave you to it.” The natural next step is simply to move away. If someone is facing a serious problem, however, the same words might sound as though help is being withdrawn, so context still matters. The expression is not uniquely Australian, but it is entirely natural in Australian workplaces and everyday conversation. Avoid the stiff “I leave you alone to do it”, which can sound deliberately distancing.

https://dictionary.cambridge.org/thesaurus/articles/having-a-lot-to-do
https://www.collinsdictionary.com/dictionary/english-german/leave