Methane sites on an old landfill below Whangarei's Pohe Island have been monitored regularly to ensure the safety of workers building a new bridge across the Hatea River.
Thirty sites in and around the former landfill are regularly checked for methane, with most showing no detectable levels of the combustible gas generated by decomposing organic material.
Since 2007, there have been three occasions when combustible levels were detected, so the risk is considered low.
Whangarei District Council spokesman Simon Weston said the main risk factors were asphyxiation - if oxygen levels dropped below 20 per cent, which usually occurs more in confined spaces - and combustion, where methane is in the 5-15 per cent range.
"Most methane detected is below or above 5-15 per cent which is the specific risk range for combustion," he said.
"On only three occasions since 2007 have levels been in the combustion range."
Mr Weston, the council's group manager for Infrastructure and Services, said landfills had a number of consequences that needed to be managed at all times - whatever work, planting, excavation, road-making, building, development or activity was taking place on them.
"Factors that need to be managed are drainage, leachate, stability, ground settlement, gas from decomposition and managing clay capping.
"Clay capping essentially seals off the rubbish from the air, allowing decomposition to take place while containing it and creating a barrier between it and people, fields and structures," Mr Weston said.
"It also contains the methane ..." These hazards are well known and managing them properly has been a priority throughout the building of the Lower Hatea River Crossing and all the other work on the former landfill.
"Management includes monitoring gas levels to ensure we have accurate information about methane emitted across the entire area including the parks, facilities and bridge site.
"We measure the methane so we can ensure that if any issues arise they can be dealt with properly.
"Measures to manage the methane include avoiding breaking the clay seal and restoring the clay cap quickly and effectively if it has to be broken," Mr Weston said.
"We also ensure that all people working on the site know the landfill emits some methane, the effect this can have, and that the correct safety gear and procedures must be used when working in the ground. Measures include installing signs to ensure that anyone doing any work in future knows to take safety precautions related to working with methane."
For the new bridge project, the council got landfill specialists in at an early stage to determine the hazards of working on the old landfill and what measures needed to be taken to avoid harm to workers, road users and the general public and ensure the safety of the bridge structure.