“Works a Treat” Means “Works Very Well”

In Australian English, if something works a treat, it works very well or proves especially effective. Treat does not mean a sweet here, so the phrase should not be interpreted word by word.

A: Did restarting the router help?
B: Yes, the Wi-Fi works a treat now.

The expression has an informal, satisfied tone and is often used when a tool, method or quick solution does exactly what was needed. Works well is similar but more neutral. In a formal report, works very well is clearer.

https://dictionary.cambridge.org/dictionary/english/work-a-treat