The recipient of the dangerous parcel from Passau in the German state of Bavaria, Rudolf Prinz, is suffering from burn-like irritations to his eyes, nose and skin after opening a parcel of cigars which released a strange liquid and vapour.
Two others, also sent from Passau, were intercepted by police before fellow councillors in Weisskirchen, northern Austria, could open them.
"My eyes, my nose, my skin are (all) burning," said Prinz, who left his post as mayor of the town just a few weeks earlier.
Austrian police, working with German authorities, have yet to identify the precise nature of the substance - but said it could involve cyanhydric acid or chlorohydric acid. Locals have been warned to be on the lookout for suspicious deliveries.
In February, the mayor of a small market town west of Vienna collapsed after eating a chocolate poisoned with strychnine, a poison often used to kill rats which can be fatal for humans as well.
A man was later arrested after traces of his DNA were found on a note left on the windshield of the mayor's car. Hannes Hirtzberger, 56, was taken to hospital where doctors placed him in
an artificial coma - from which has yet awakened.
The suspect, who maintains his innocence, owns a vineyard on which he had wanted to build a spa hotel only for the mayor to turn down his application.