The New Zealand Transport Agency (NZTA) had contracted Downer and MWH to realign the roadside to create a public cycleway.
Yesterday, Andrew Beattie, of MWH, uncovered 3m of the cobblestone channelling buried half a metre underground, adding to the 19m that was discovered last week by the contract workers.
''I had a bit of a scratch around and found that,'' Mr Beattie said. Independent archaeologist Jill Hamel, of Dunedin, was called to inspect the cobblestones.
She would need to complete a report before it was officially recorded, she said, but the stones were probably late 19th-century water channelling.
Disc channels for the city's water management had lined Castle St and dated back to the 1880s to 1890s, Mrs Hamel said.
''It was made about the time when Castle St was first established.
''It's part of the city jigsaw,'' she said.
NZTA project manager Simon Underwood said the discovery was unexpected, but on certain jobs it had in place what was called an ''accidental discovery protocol''.
''Some jobs we expect to find something and have an archaeologist on standby.''
The New Zealand Historic Places Trust was notified of the discovery and would be given a copy of Mrs Hamel's report.
The discovery did little to slow the NZTA's progress on the main highway heading south, and today the cobbles would be covered over and the job carried on, Mr Underwood said.
