The first of six new ferries to operate in Sydney Harbour will be in the water within two years.

Transport Minister Gladys Berejiklian has issued tenders for the construction of the ferries, which, she says, will be some of the fastest in the fleet.

An artist's impression of one of the six new inner-city ferries.

Work on the ferries, which will replace some of the First Fleet catamarans, will start next year.

They will look similar to the First Fleet vessels, with their boxy shape, but will have a second upstairs outdoor viewing area.