Your inverter shuts itself down on purpose. It’s a safety requirement called anti-islanding protection, sometimes called backfeed protection, and it’s mandated under Australian Standard AS/NZS 4777.2.

Here’s why it exists. When the grid goes down, the power lines in your street are de-energised and network crews go out to work on them. If your solar system kept pushing electricity into those lines, it would be dangerous for anyone working on the network. So your inverter detects the loss of grid power in fractions of a second and disconnects.

It isn’t a fault in your system. It’s the law.