Thirteen years after the Sept. 11 attacks forever changed the New York skyline, officials say most work at the World Trade Center complex is on track and on budget.

The complex sits on the site of the 2001 terrorist attacks and plans include four buildings, the 9/11 memorial and museum, a shopping centre, and a transportation hub, known as the oculus. The memorial and museum have opened, as have parts of the transportation hub.

Two of the four World Trade Center buildings have been completed, and five million square feet of office space has already been leased.

State and city officials held a news conference Tuesday from the 67th floor of 4 World Trade Center, which opened in November.

A completed World Trade Center complex will create a vibrant district and make Lower Manhattan a business and shopping destination, they said.

"It will be a vibrant 24/7 downtown for people around the world," said Port Authority Vice Chairman Scott Rechler. "In the aftermath [of the attacks] that would have been hard to predict.

"Although costs soared and plans changed for 1 World Trade Center at the beginning of construction, it will open sometime this fall, Rechler said.

"We owed it to all those who perished" to persist and complete the building," Rechler said.

The centre and a transportation hub will open in phases and may not have a grand opening, he said. The centre will open when media company Conde Nast moves into its 1.1 million square feet of office space, he said. The building will have a 60 per cent occupancy rate.

Technology company MediaMath signed a lease earlier this summer for 100,000 square fee of office space in 4 World Trade, developer Larry Silverstein said.

Silverstein said his company is obtaining financing to complete 3 World Trade Center by early 2018.

Its construction was halted last year because the building lacked an anchor tenant.

He said he plans to open the tower by early 2018. The new date comes after media investment company GroupM signed a 20-year lease to occupy nine of 80 floors.

"Both MediaMath and GroupM represent the technology, advertising, media and information industries that are currently driving the city's economy," Silverstein said. "Their decision to relocate to the World Trade Center proves that New Yorkers were right to bet on downtown."

Plans are on hold for construction of 2 World Trade Center while Silverstein seeks tenants.