Andrew Lezala, Metro's chief executive, says ''high-speed rail in Australia is just a dream'' unless it can be constructed at less cost than the world's other high-speed railways.

''In Australia - in any country, really, unless there's a huge political objective - unless we can build a high-speed line for $10 million per kilometre, it's not going to be commercially viable and therefore it's not going to happen,'' Mr Lezala told the Engineering Futures Forum at Swinburne University of Technology on Thursday night.

But Mr Lezala said there was a solution: his own concept, eight years in the making.

Mr Lezala, who is an engineer, proposed building a high-speed, elevated light rail that would run beside the freeways between Melbourne and Sydney.

At 350km/h, the Melbourne-Sydney journey would take just two hours and 29 minutes. ''Melbourne to Sydney in just under 2½ hours is very competitive with an aircraft - Southern Cross to Central in that time,'' he said.

The Melbourne to Sydney air corridor is among the five busiest in the world. Airport congestion contributes to about one in five flights between the two cities being late. Mr Lezala said light rail was the key to his proposal's affordability, because it would enable the railway to be elevated and built alongside existing freeways, avoiding the need for expensive land acquisition.

The government is studying the feasibility of a high-speed railway between Melbourne and Brisbane. The study has found that building the railway would cost between $61 billion and $108 billion and involve laying more than 1600 kilometres of new standard-gauge, double track.