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.

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.

"The new vessels will be some of the fastest on Sydney Harbour, and will be a modern version look and feel of the very popular First Fleet vessels that are loved by our customers, visitors and tourists," Ms Berejiklian said.

"Each ferry will carry up to 400 passengers and some of the great new features include large outdoor viewing areas, around 90 more seats than the current First Fleet vessel, two wide walk-around decks, Wi-Fi access and real-time journey information," the minister said.

There are 28 vessels in the Sydney Ferries fleet, though the private operator also leases three charter boats.

The new vessels will be the first built and acquired for the fleet since four SuperCats were acquired in 2000 and 2001. The six new vessels will operate on inner harbour routes from Watsons Bay to Cockatoo Island.

Ms Berejiklian said the government was continuing to look at options to serve the Parramatta River.