The cows will be joined by 6000 people expected to attend a picnic on the coathanger as part of the Breakfast on the Bridge event, the centrepiece of the Crave Sydney festival.

To help the cows acclimatise, a hectare of Kikuyu turf from Pitt Town in Sydney's far north-west will be transported to the heart of the city and laid over the tarmac.

Some commuters may have eaten breakfast in their car on the bridge before, but not on a picnic rug and never beside the source of that breakfast staple, milk.

"For some people that come from the city, it's going to be an eye-opener that milk comes out of an actual cow and not a carton," said Danielle Krix, the farm manager at Hurlstone Agricultural School.

"I suppose the cows just get used to the railway and if we're preparing animals for a show we handle them quite a lot and make sure there's a lot of noises around them."

Evergreen Turf is the company responsible for trucking in the turf to cover the bridge roadway and its chief executive, Dean Holden, said it would take about eight semi-trailers to transport it and three hours to lay it.

"Pretty much the size of a rugby field is the amount of turf we are laying and we have about 100 people assisting us," Mr Holden said. "You never roll up turf, either, so we've had to work out how to do that efficiently and get the bridge back open in the timeframe as well.

"Three o'clock in the morning is always fun to be doing a bit of work … but while looking over Sydney Harbour it will be a magnificent experience."

Ticket holders will be able to milk the 15 cows. Breakfast on the Bridge will run for two hours from 6.30am, with the bridge closed from 1am to 1pm.