The City became aware of the problem after removing a dying Moreton Bay fig from the corner of Stevens and Brennan Street. Investigations showed that the decay could have been caused by substances it had absorbed through the soil.
Fremantle technical services director Peter Pikor said there remained no threat to local schoolchildren.
‘Right from the start the City saw the issue as isolated and not a risk to public health,’ he said.
The DER will now review the DSI and return recommendations for the site to the City. Mr Pikor said the City was using consultants to draw up a site management plan.