The New Zealand Transport Agency's annual spring inspection began last Monday and is scheduled to continue until Friday - it's one of two annual inspections carried out on the bluff, which is about 500m long.
Inspections have been carried out regularly since 2001 after 10,000cu m of rock fell on to the highway in September the previous year.
Opus associate project engineer and Nevis Bluff manager Reece Gibson said the largest failure at the bluff was in 1975 when 30,000cu m of rock came down.
Larger rock falls had natural ''return periods'' of between 13 and 23 years without intervention.
However, the regular inspections by NZTA contractors aimed to prevent future failures.
Along with the twice-yearly ''spring clean'', helicopter inspections were done monthly to identify and rectify any urgent issues.
The present work includes aerial mapping of monitoring pins to check rock movement and, where possible, bolting rock back into place.
Where the rock quality was poor, contractors removed it either through blasting, sluicing or ''air bagging''.
On Friday afternoon, an air compressor was airlifted on to the bluff before air bagging removal, which began on Monday.
Mr Gibson said the air bag was similar to an ''industrial hot water bottle'' which would be inserted into cracks in the rock and then inflated to remove the loose or at-risk rock.
Motorists could expect delays of 10 minutes between 8am and 5pm until Friday while the work was being carried out.
