By studying historical records for thousands of water bores across NSW, researchers from the NSW Department of Environment and Climate Change and the University of NSW have shown that salinity is traceable to rising groundwater levels.

This means that the salinity crisis that threatened thousands of farms in the 1980s and '90s is now in retreat as the land dries out as a result of drought and climate change. Higher groundwater levels mean more water interacts with friable, sandy soil and forms the crust of salt that can accelerate erosion and destroy agricultural productivity.

"The mistake we made in the past was to assume that the kind of rainfall we were seeing from the middle of the 20th century was normal, whereas it was actually quite wet by historical standards," said Professor Ian Acworth, a University of NSW hydrologist who worked with the environment department researcher Aleksandra Rancic.

Long-term rainfall variability, separate from human-induced climate change, is expected to mean that slightly drier conditions will up to the middle of the century, followed by another period of higher average rainfall.

"Dry land salinity is not going to be a problem as much in the first half of this century than it was in the last half of the previous century," Professor Acworth said.

"Another way to look at it is to say we probably have about 40 years to repair marginal land."

The standard response to encroaching salinity has been to plant trees, because chopping trees down to extend grazing land has been seen as the prime cause of the salinity crisis. But the groundwater research strongly suggests that land-clearing has played a secondary role.

Another factor is that Australia's soils may be particularly prone to going saline, Professor Acworth said, given that many countries have cut down most of their native trees without experiencing salinity.

The records from water bores show that an extended dry period lasted from the beginning of the "Federation drought" in 1895 until just after World War II. There was then a shift to higher rainfall, and with it increasing reports of farms being overtaken by salinity. The crisis appears to have peaked in the '90s, with about 2.5 million hectares of land affected.