The spraying, which will continue when weather conditions permit, targets the most significant areas of fairy grass infestation over a 500ha area of the lake.

One of two sections of the lake was sprayed yesterday.

City of Ballarat sustainability director Ian Rossiter said while spraying was not the preferred action, it was necessary to control the fairy grass inundation.

``The Learmonth community can be assured that council has been working together with residents and state government agencies to ensure the extent of the section to be sprayed and the timing,'' Mr Rossiter said.

``It is important that these factors are taken into account to ensure we optimise our opportunity to control the fairy grass before the seed heads become an issue.''

Helicopters with global positioning unit controls to accurately spray target areas are being used to spray the lake.

Approvals from Department of Sustainability and Environment, Department of Primary Industries and Glenelg-Hopkins Catchment Management Authority were obtained to spray Roundup Bioactive herbicide.

Trial plots and a narrow buffer zone around the lake edges are not being sprayed to prevent drift to adjacent crops, trees and desirable vegetation.

Council and DSE officers and resident representatives have been conducting fortnightly site inspections to determine the optimal timing of the spraying and to monitor the extent of the fairy grass invasion, impacts of spraying, grazing and aerial seeding.

Council's long term treatment plans for Lake Learmonth include strategic planting or indigenous species that can tolerate and compete with fairy grass.