In recent weeks a number of motorists have noticed that the casuarina trees are starting to die. And not just here and there either but in a great swathe along the median strip from the Five Islands Road south as far as the Fowlers Road overpass.

Helensburgh's Noel Kendall travels along that stretch of road and he has noticed something is not quite right.

"It started to be noticeable two or three weeks ago," Mr Kendall said. "They're looking deader and deader every time you drive past them.

"They look like someone's gone about spraying herbicide or something underneath around the base of them and the spray has gone up into leaves of the tree."

Mr Kendall said the condition of the trees was a shame not only because of their normal beauty but also because they provide a screen from oncoming traffic and their headlights.

The median strip and the trees are the responsibility of Roads and Maritime Services.

A spokesperson from RMS said that they had advice from an arborist suggesting "recent weather conditions of a dry and warm late winter-early spring has turned the branches of the casuarina trees in the median brown".

"It is expected the recent wet weather will improve conditions for the trees and they should return to their normal green appearance."

An arborist consulted by the Mercury said that the recent drought conditions was one of several explanations for the poor health of the trees.

Warwick Barley is an arborist consultant with Allied Tree Consultancy and he has driven past the fading casuarinas several times in the past few weeks.

He said their condition was a surprise because casuarinas are chosen for use in urban areas because they are a "pretty tenacious species".

"That's what raised my eyebrows when I saw that group dying away, because I thought that was very uncharacteristic," Mr Barley said.

"And I noticed that the grass beneath them had suddenly browned off, which generally doesn't happen, even during droughts."

Mr Barley said the cause could be due to a lack of recent rains or an overzealous contractor spraying a too-strong batch of herbicide on the undergrowth.

Whether the trees have been poisoned, are suffering from lack of water or have some other ailment, Mr Barley said that could only be determined by testing.

"We see a lot of plants which show symptoms similar to poisoning, that is why you have to do further examination and testing to be able to tell what's actually going on," he said.

"Whether you've ... a plant that has died from a root pathogen or from drought or from poisoning, they all ... have similar symptoms."