An Auckland mother was left frightened and angry after a rock she believes was thrown from a motorway overbridge struck her windscreen

Danielle Williams was travelling from the supermarket to her Te Atatu home at midday today when she says she saw a "group of kids" standing on the Huruhuru Road overbridge crossing the Northwestern Motorway in Massey.

She thought nothing of it until she heard a massive bang.

She believes a rock was thrown from the bridge, striking the windscreen.

Her aunt and 11-month-old son were both in the car and got a huge fright.

Danielle Williams says it's lucky the rock didn't crash through the windscreen, and has one message for the people who threw it - don't do it.