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New Zealand Transport Agency senior network manager Mark Pinner said yesterday about 150cu m of rock and material, including a 20-tonne boulder, fell on to the highway at the slip site on Tuesday night.
More than 200mm of rain was recorded at the site between Monday morning and Wednesday morning, almost half of it on Tuesday night.
Heavy rain continued yesterday. Mr Pinner said the NZTA's geotech experts were on site yesterday assessing the slip face.
''They are concerned with the ongoing movement of rock and material near the top of the slip face which has prevented us from being able to clear the road,'' he said.
''The safety of road users and our contractors will always be our top priority.''
Mr Pinner said contractors would be on site at dawn today to undertake a review of the slip face and when it might be safe to clear and reopen the highway.
''We will also need to undertake some sluicing on the lower section of the slip where there remains some loose material.''
The initial slip on September 10 was estimated to have dumped 40,000cu m of debris on the highway and road closures were in force between 6pm and 8am at the time of the latest slip.