Extra legroom seats to Sydney

I am travelling to Sydney in economy - I thought I read somewhere before that you can't buy these seats in advance to Sydney - does anyone know if thats true. I spoke to the premier line - they weren't sure. At present it looks like they are all taken and are not sure if any will be available at the airport.

Does anyone know how they work as really hoping to get these seats?

Thanks

check this page for details

Brilliant - thank you. Knew I had heard it somewhere