The NZ Transport Agency (NZTA) says protecting competitors from possible rockfalls is behind its decision to exclude the Homer Tunnel section of State Highway 94 from this Saturday's (26 January) Milford Sound to Te Anau cycle race.

Around 20 kilometres has been trimmed from the 120km race distance due to the rockfall risk at the western entrance of the Homer Tunnel. This rockfall situation has seen the road closed at night from the Hollyford turn-off to Milford Sound since early December.

NZTA Southland Area Manager Peter Robinson says we understand this move will disappoint some, but we believe with the current rockfall risk on this section of the road it's the right decision. Although we have been working together on a contingency for the race it was with the hope that the planned route might be available. Unfortunately we have had to confirm this week that access to the western approach of the Homer Tunnel for the race is too high a risk.

"Cyclists would have limited ability to get to safety, even with the safety systems we have in place and that is a risk we are simply not prepared to take and while obviously disappointed, the race organisers understand our position."

Another factor was the western tunnel entrance and the highway for several hundred metres towards Milford Sound, which will be transformed into a construction zone by race day, with work underway on erecting a temporary 60 metre portal extension on this side of the tunnel. This job is expected to be completed by mid February.

The portal extension is to help protect vehicles from damage from small pieces of scatter rock that could fall from eroding rock above the western tunnel entrance Mr Robinson said.

The new start point for the second running of this annual cycle race is the Homer Hut Carpark about 1km on the Te Anau side of the Homer Tunnel entrance.