Mary Veronica's mouth is nearly two metres in diameter.

The massive tunnelling machine, named after one of Sydney's first female mayors, is about to start chewing her way through 12 to 15 metres of earth a day to create a 2.4-kilometre long tunnel that will floodproof Green Square.

It was the dream of early planners to build a canal that would connect the low-lying swamplands of the inner city to the Alexandra Canal.

That's finally happening, but below ground. Costing an estimated $80 million, the culvert will be nearly two metres high in some parts. It will be wide enough for two cars to drive side by side where the drain meets the Alexandra Canal.

The City of Sydney said the underground drain would be big enough to empty 300 bathtubs a second – about 30,000 litres – preventing the floodwaters that have regularly trapped drivers over the years. Two months ago, emergency service personnel waded through chest-high water to rescue people from submerged vehicles.

This tunnel will divert floodwaters away from Green Square – one of the fastest growing residential areas in NSW – and allow construction of the Green Square Town Centre, the first new town centre built in central Sydney for more than a century.

The Green Square machine was named in honour of Mary Veronica Nelson, who was a mayor between 1946 and 1947.

Mrs Nelson's granddaughter, Janine Loyzaga, said her grandmother had been a woman of strong views, strong opinions and a leader in community service in a time when it was unusual for a woman to be any of those.

"She would have been proud and appreciative to have her contributions to her community recognised," Mrs Loyzaga said.

Tunnelling will begin in June and is due to finish by the end of 2017.