Organisers of the fair on the Town Moor said they had now dealt with standing water and thick mud, which had made setting up stalls and rides difficult.
One of Europe's largest, the fair was due to be officially opened by the Lord Mayor on Friday but will now open at 13:30 BST.
Northumbria Police are warning there will be no on-site parking.
A statement issued on behalf of Newcastle City Council, the city's Freemen and the Showmen's Guild, said improvements had been made to the site.
"Those involved in the management and restoration of the site have been working non-stop to improve the conditions," the said.
The Freemen's vice chairman, David Wilson, said: "We brought in massive machines to even out the surface.
"We have also brought in large quantities of bark chippings and straw to soak up some of the water as well as drafting in two submersible pumps to collect water which can then be moved off site."