The great willowherb has been discovered growing in North Canterbury for the first time.

An invasive weed previously not found in New Zealand has been discovered growing in several areas in Canterbury.

Biosecurity New Zealand spokesman John Brightwell said the great willowherb weed had been detected at five sites in and around Lake Pegasus in the Waimakariri District and near the Kate Valley landfill, about 40 kilometres north of Lake Pegasus.

A field team was searching all likely places in the wider Canterbury area to determine the extent of the invasion, he said.

"We're in the early stages of determining the severity of this incursion. Once we've determined this, we'll work closely with Environment Canterbury and the Department of Conservation to develop an appropriate response."

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Anyone who thought they had seen the weed should call the Ministry for Primary Industries' biosecurity hotline, he said.

"It's extremely important that members of the public do not attempt to remove this weed on their own. They may mistake it for other, similar-looking native plants and, just as importantly, they may risk spreading the seeds."

The weed is known for its aggressive growth and can crowd out native wetland plants. The weed currently poses no risk to fresh water fish or agriculture.

The great willowherb is a flowering plant also commonly known as the great hairy willowherb. The highly invasive weed is commonly found in parts of North America and in Victoria, Australia.