Costas has been engaged in consulting engineering for over 35 years in the fields of water supply, wastewater engineering and stormwater drainage and treatment. He specialises in the design of pipeline, pumping and drainage systems and has been responsible for the design of numerous projects for local councils, commercial clients and developers here in New Zealand. He has also worked in UK, southern Africa and in the Pacific islands.

Costas is a Chartered Professional Engineer (NZ and UK), Member of the Chartered Institution of Water and Environmental Management (UK), and is a Chartered Environmentalist (UK). He is a Senior Engineer and Principal of Tonkin & Taylor.