During heavy rain on Tuesday, the Waiho bailey bridge was swept away, cutting off access between Fox Glacier and Franz Josef.
NZ Transport Agency systems manager Pete Connors said the initial timeframe aimed to complete repairs within 10 days of the damage, but repairs are now likely to take 17 days.
"We started off with a pretty ambitious plan on the best information we had, and as more information has come to hand and we've developed the methodology, that's our best guess at the moment," Mr Connors said.
Problems with site access and materials were some of the complexities which had slowed progress, he said.
MetService has issued warnings about more heavy rain for Westland tonight.
Severe weather could push the repairs timeline further back and this could cause havoc at the Easter break, Mr Connors said.
Waiho's compromised south side stopbank was also a concern ahead of the rain warning, he said.
"Because of the build-up of material on the river bed from the flood, it could take two to three months for this stopbank to be reinstated to its previous protection levels."
Since making the original plan for the bridge repairs, the Transport Agency has gathered more information, completed more planning and worked on design.
The new bridge will been upgraded to allow all vehicles categorised as Class 1. This now includes "heavier trucks, not just cars and buses as initially planned".
