Recent stormy weather has caused considerable setbacks to remedial work at the Diana Falls slip site, where about $5 million has been spent by the New Zealand Transport Agency to date.
Progress at the slip site to reopen State Highway 6 through Haast Pass to two-lane and 24/7 traffic has been hampered by rainfall and strong winds which have lashed the region.
NZTA senior network manager Mark Pinner said in the past month the highway from Haast township to Makarora had been closed on two occasions.
''The weather has brought rocks weighing up to three tonnes and other material on to the road, causing irreparable damage to some of the anchor post bases that had been drilled into the slip face to support the rockfall fences that are being installed.
''Unfortunately, some of this work has had to be redone as it was damaged before we could begin to install the first of the fences.''
The material for the rockfall fences had arrived from Switzerland and Australia, but it was now unlikely installation could begin until next month, with a completion date in late September.
Mr Pinner estimated about 18 days of work had been lost at the site, both from damage to the work already completed and the time when it was unsafe for workers to be on the slip-face.
''The flipside is it has brought down a lot of rock that would have eventually made it way down the mountainside. Longer term, this should mean we will have a more stable site.''
NZTA was reviewing the work programme to see if any changes were possible to get the highway opened any quicker.
''We always knew it would be a difficult time to be working on site over winter but it has always been our goal to have the highway back to two-lane and 24/7 traffic in time for the next tourist season.''
Once the first rockfall fence was installed at the top of the slip-face, it would provide a greater level of protection and help with progress as the remaining two fences were installed.
