The lippia species Phyla canescens from Argentina has invaded more than 5 million hectares of the Murray-Darling Basin, but the NSW Government has resisted calls to ban the weed and its less invasive cousin, Phyla nodiflora.

Described by the botanist Matt Macdonald, who did his doctorate on it, as the worst weed west of the Great Divide, Phyla canescens is often wrongly labelled as Phyla nodiflora and sold in nurseries, according to a new manual published online by the National Lippia Working Group.

Sydney Water gives Phyla nodiflora - the less invasive of the two species - the best waterwise rating on its plant selector web pages, and says it is suitable for planting across large areas of the city and the Illawarra.

But Dr Macdonald found the falsely labelled weed at stores and nurseries in NSW and Victoria while working on his PhD at the University of New England last year. He found more on sale in a Perth nursery last week.

He said he had never seen genuine Phyla nodiflora on sale.

The two lippia species were very difficult to distinguish and both should be banned, he said.

"If you haven't looked closely at them, they can be hard to tell apart. Even I have to umm and ahh about some of them."

The Moree cattle grazier Bruce Southeron, who reckons he has lost more than $2million in income over the past decade, also wants both species banned.

In his 30 years on his 4450-hectare property, Old Dromana, 70kilometres west of Moree, he has watched lippia form a knotty killing mat over his paddocks, forcing a reduction in his stock from 2500 to 1500 head of cattle.

Where 1000 breeding cows once ran, only 600 can now thrive, cutting the number of calves he can sell by 400 - a loss of $200,000 in income each year.

The precious publicly owned Macquarie Marshes are also being progressively strangled by the weed, which costs an estimated $1.8billion a year in environmental losses such as erosion.