Pollution in Botany bay

Clean-Up Notice Issued Over Controlled Discharge into Botany Bay
The NSW Environment Protection Authority (NSW EPA) has issued a Clean-Up Notice (2 April 2014) to Caltex Refineries (NSW) Pty Ltd (Caltex) under s. 91 of the Protection of the Environment Operations Act 1997 No. 156 (NSW) after Caltex discharged oily water from its Kurnell premises into Botany Bay during heavy rainfall on 24 March 2014, resulting in the oily water moving to the shoreline between Yarra Bay and Congwong Bay in Botany Bay on prevailing winds and settling on rock platforms and in the sand at Congwong and Little Congwong Beaches. NSW EPA chief environmental regulator Mark Gifford stated that Caltex the controlled release was part of the company's emergency procedure for stormwater management during heavy rain; however the incident has had an "unacceptable" impact on the local environment and the clean-up notice is "the first in a potential series of clean up notices to be issued in relation to the immediate impacts of the incident". According to Mr Gifford, the clean-up notice directs Caltex to undertake an ecological assessment program, with NSW EPA to select a preferred ecological practitioner "to ascertain the nature and extent of the pollution incident on the intertidal ecology of the rock platforms and beaches at Congwong and Little Congwong Beaches and assess the recovery from the pollution incident". The practitioner will also be required to "submit written reports to [NSW EPA] and make recommendations regarding the need for any further actions required". Mr Gifford also advised that NSW EPA is conducting a detailed investigation into the incident.

And this will be noticeable above the thin layer of kerosene that normally floats around the northern half of the bay?

Microbes eat the stuff and render it harmless, but instead, they'll probably go spraying all sorts of weird dispersing detergents, and no-one will see the real hidden damage that occurs as a result.