Biosecurity SA says thieves are targeting South Australia's first legal hemp crop and has issued a reminder that the product will deliver "absolutely no high".

Key points: The first legal hemp crops are being harvested in South Australia

The first legal hemp crops are being harvested in South Australia A State Government agency and farmers say thieves are targeting the plants

A State Government agency and farmers say thieves are targeting the plants They have warned the product will not make people "high" and pesticides could make users sick

The South Australian Government agency's rural chemicals operations manager Michael McManus said he had become aware that some hemp producers were facing unwanted attention on their crops.

The first commercial harvest of industrial hemp crops is underway.

"It is just not worth the risk to try and steal them," Mr McManus said.

"Industrial hemp should not be confused with medicinal cannabis or marijuana.

"This crop is essentially just like any other agricultural crop such as wheat or barley.

"Depending on the final purpose of the industrial hemp crop, spraying for insects and weeds can occur, so anyone thinking of stealing industrial hemp plants to either consume or smoke will experience absolutely no 'high'.

"They should also be aware that if crops have been sprayed, any consumption could in fact lead to a risk of becoming seriously unwell due to potential pesticide exposure."

Hemp has very low level of THC

There are 11 producers licensed to grow industrial hemp in South Australia, two of those are licensed processors.

Industrial hemp was legalised in South Australia in 2017.

Industrial hemp is defined as containing less than 1 per cent of the psychoactive chemical tetrahydrocannabinol (THC) which gives people the "high" associated with marijuana.

A hemp plant growing on a farm. ( File: Rachel Riga )

Hemp can be used to make clothes, cosmetics and building materials.

Mr McManus warned anyone caught trespassing at crop sites could be charged.

"Police are aware that industrial hemp thefts are occurring and are monitoring the situation," he said.

Alice Morley, who works on a hemp farm at Hindmarsh Tiers, on the Fleurieu Peninsula, south of Adelaide, said people had repeatedly tried to steal their crop.

She warned people not to smoke it since pesticide had just been added by helicopter.

"You'd have to smoke a truckload and you'd get very sick before it had any effect on you," Ms Morley said.

"But Rob, the owner, is having a lot of problems with people who think they can get high on it themselves or thinking they can sell it on pretending it's the real stuff.

"There's been prosecutions, people are being arrested, regos being taken, there is surveillance and it's pretty damn good.

"The cops are coming down from town and people are going to be caught so please respect farmers' biosecurity in this.

"Not only in stealing plants are you affecting someone's livelihood, you're going to make yourself very sick once this stuff has been applied, especially in the next seven days if you're going to be smoking any of it."

Some of the products hemp can be made into, such as muesli and rope. ( ABC News: Loukas Founten )

First crops being harvested in SA

The first commercial hemp was harvested in South Australia last week near Bordertown, south-east of Adelaide, a few weeks earlier than expected.

Mick Anderson from Good Country Hemp — the business the hemp seed is contracted to — said "jokes about [cannabis]" aside, it had been a very exciting journey.

"It will be the first hemp crop that's ever been harvested in South Australia so we're pretty excited about that," he said.

Mr Anderson said as a health food hemp contained plenty of benefits, but it was also a handy cash crop for farmers.

"I think as the industry grows you'll see more and more hemp crops through the countryside and in five years time it will be a commonplace crop," he said.