Now several of the gum trees that filtered that view have been poisoned and an investigation is under way to determine if it was an unfortunate mistake or wanton vandalism.

The eight poisoned blackbutt gums form a mini-forest around the complex on Homer Street which looks out over the Cooks River, Marrickville Golf Course and through to the CBD.

The brown gums are conspicuous among the canopy of green, and neighbours said they had noticed the change in colour over the last few weeks.

"For God's sakes," one neighbour said. "I did think to myself they were dying. It's especially sad when they are big trees like that."

"I don't think you would be a sceptic to say that something bad had happened to them, you would be truthful," another said. "I think those big gum trees are pretty fragile things."

A spokeswoman for the strata that looks after the site, Bonded Strata Management, said the trees were "loved and adored" and described the poisoning as a "tragedy".

The strata believes the trees were accidentally poisoned when land regeneration works were undertaken in April.

A hired "bush regeneration specialist" used strong chemicals to kill weeds and unintentionally infected the gums, the spokeswoman said.

Some residents living in the apartment complex said they were not aware the gums had been poisoned, others had noticed they had turned brown but did not want to comment.

Canterbury Council's general manager Jim Montague said an investigation at the site was still under way and tissue samples were being taken from the trees to determine the cause of poisoning.

"Determining the cause of and intention behind any tree poisoning can be difficult," Mr Montague said.

"If the trees have been intentionally poisoned, and the perpetrator can be identified, the council will prosecute those responsible to the full extent of the law."

Nature Conservation Council of NSW chief executive Kate Smolski said old and big trees in urban environments were a particularly important habitat for wildlife.

"Removing these trees has a big impact at the local scale because there are so few of them left in many parts of Sydney," Ms Smolski said.

"Large trees are wildlife hotspots where birds come to feed on the nectar and insects. When they are gone, a whole array of animals goes with them."

While there is no evidence the poisoning was intentional, Sydney councils have found it challenging to catch and prosecute vandals who kill or remove trees illegally.

Coastal councils have instead used banners or signs proclaiming versions of "trees shouldn't die for a view" and "these trees have been wilfully destroyed by selfish vandals" to block the views created. Rewards have also been offered for information.

North Sydney Council, which had a banner in place for more than two years at Gore Cove Reserve, said there had been a slight decline in tree vandalism since the sign strategy was introduced.

Waverley Council also adopted a "two for one" policy, where every tree illegally killed or removed was replaced by two trees.

The results from the testing at Earlwood are due next week. The council has not decided if fines will be issued if the trees were accidentally poisoned.

A council spokesman said the gums would not be cut down unless they started to pose a danger.