MetService meteorologist Clare O’Connor said there were about 150 lightning strikes in the Oamaru vicinity and more than 15mm of rain from 3pm to 4pm.
"That’s going to be really noticeable," she said.
Herbert received 10mm of rain in the same period.
The high rainfall was associated with the thunderstorm, she said.
A lot of moisture was contained in the large clouds, which squeezed out in small, concentrated areas.
She said thunderstorms were typically on a smaller scale than regular rain clouds but had more energy involved.
Despite the rainfall, Ms O’Connor said it was important to keep up to date with Fire and Emergency New Zealand fire seasons.
This weekend is expected to be a "cool, windy weekend", with rain tomorrow.
Today is expected to be a bit fresh.