The cavity is about 250cu m in volume, and about 5m under the surface.
It was discovered by contractors drilling proof bores at the planned location for the new Riselaw Rd bridge piles.
Supplementary drilling exposed the extent and size of the void.
New Zealand Transport Agency projects team manager Simon Underwood said the hole would increase project costs and delay the overbridge construction.
Added costs would not be known until further design work relating to revised foundations had been done, he said.
''We had planned on completing the bridge works this year, but this is now likely to be closer to April next year.''
Mr Underwood said the cavity did not extend underneath the highway and there were no houses or buildings ''anywhere near it'', so it was not a matter of public risk.
''It is an engineering challenge.''
The cavity was part of a wider, complex geological picture of the underlying ground conditions at the bridge, and the design of the bridge and embankment foundations had to be ''managed'' to suit, he said.
The cavity is within a layer of sandstone, which is 5m-7m below the original ground level.
Contractors had previously excavated down to the sandstone layer and placed 2m of imported rock fill on top, and planned to add another 2m-3m of fill.
Mr Underwood said the hole was about 3m below the top of the sandstone layer.
''We would have preferred the sandstone layer to be `feature' free, but we are far better off knowing about this now,'' he said.
rosie.manins@odt.co.nz#versham
