Essential earthquake-strengthening work will start on Wellington's Hataitai bus tunnel in early November, and is estimated to take up to six months.
The project is out for tender and carries an engineering estimate of $850,000 and the overall cost will be between $1.1 million and $1.3 million.
Wellington City Council was commissioning the work because structural assessments have shown that while the tunnel itself was likely to withstand a reasonably large earthquake, the adjacent hillside and old tunnel entrances could come down.
A start date and work hours would be agreed once a contractor is appointed next month.
The council said bus routes would remain unaffected during peak hours.