A Sydney wetland has turned pink in an unusual sight that left a top scientist on a mosquito survey bemused.

Associate Professor Cameron Webb from NSW Health Pathology was surveying Sydney Olympic Park yesterday when he came across a pastel pink pond.

The wetland was in an "out of the way spot" within Bicentennial Park on the edge of a mangrove, a critical habitat that helps protect land from erosion.

I've never seen that but I do remember a time long ago when the fountain outside our town's police headquarters turned bright blue.