Motorists will not drive through the Manawatu Gorge until late  next month or early September, but when they do it will be back on a rebuilt State Highway 3 open both ways.

NZ Transport Agency announced the new timeframe for the reopening of the gorge road last night, saying crews were working around the clock to replace damaged bridges.

A week ago flooding washed away the slip material, which  carried a temporary road that allowed motorists to go past construction work on the damaged bridges.

With that dirt gone there is not room for traffic to get through the gorge until the bridges are rebuilt.

"The team are now working within the width of the existing road,'' agency Palmerston North state highways manager David McGonigal said.

"(This) means we now need to build the bridges from either end, rather than from the sides.

"To accomplish this, we've brought in one of New Zealand's biggest mobile cranes, which can lift 450 tonnes.''

The Manawatu Gorge road has been mostly  closed since a slip on August 18.

That was the first in a series of slips, including a large one in October, that were not fully cleared until late May.

SH3 was then reopened, with motorists using the temporary road past the construction site, until last week's flooding.

Mr McGonigal said the lower structure of the bridges was effectively completed with the curing of the concrete now under way, and in  the next few weeks  the bridge beams  would be laid and grouted into place.

"We were always conscious that the temporary road was just that  temporary  and although we would have loved mother nature to have spared it for a little longer, our bridge crews are working as quickly as they can to get the road open for good.''

Mr McGonigal said the loss of the temporary road was disappointing, but the agency's  top concern was safety. Crews were making strong progress on site  during  the  improved weather.

He reminded motorists to take care on the alternative routes, and to strictly observe speed restrictions.

For more information about the Manawatu Gorge, visit www.nzta.govt.nz/manawatu-gorge

- © Fairfax NZ News