Four of the 16 plane trees - which have lined the carriageway through the Octagon for more than 100 years - have not come into leaf this summer, and others are showing signs of infection.
Council community and recreation services manager Mick Reece said, when contacted, he was "seriously worried" the trees had fallen victim to a new, as-yet unidentified fungal root disease.
"We are scared of this escalating now . . . it may be one [an infection] that isn't manageable," he said.
"It looks like what it is: a problem."
Council staff had sought specialist advice and were waiting for test results, due next week, before deciding what to do, he said.
It was possible the disease would be untreatable and would spread.
At worst, all 16 trees would die and have to be replaced with another type of tree, he said.
The trees had a history of problems with anthracnose, a fungal disease considered the "common cold" of trees, over many years, he said.
That disease could cause new shoots to fail, and stunt growth, but the trees recovered with pruning.
The trees usually came into leaf in November, but four had failed to do so this year despite regular rainfall and pruning, he said.
"This year, we have not had the recovery . . .
"This is different."
Other trees in George St, near Knox Church, and Princes St were also showing signs of the disease, raising the possibility cross-contamination of the trees had occurred through pruning equipment used on each, he said.
The infected trees were not dead, with small amounts of leaf and sap under bark, but council staff were "very, very concerned", he said.
One possibility was the plane trees were infected with the plane tree equivalent of Dutch elm disease, although that would not be confirmed until the test results were in, Mr Reece said.
The disease devastated elm trees in Europe and the United States last century and was found in Auckland in 1989.
The plane trees, planted in 1891, line both sides of the central carriageway of the Octagon.
Four at the southern end of the carriageway were all but devoid of foliage - three on the lower side of the Octagon and one across the street.
Several others were also showing signs of disease.
A report containing an update on the trees' condition would be considered at the community development committee meeting today.
Mr Reece said the council would have to decide a course of action once test results were in.
Other measures might also have to be considered, including sterilising all pruning equipment, which would come at extra cost.
Worries about the trees' health come just months after one of the old plane trees was replaced, at a cost of $4000, in August last year.
The roots of the original plane tree were damaged when the Octagon was upgraded in the late 1980s, and it was affected by anthracnose.
