Worksafe NZ outline and define planning entry and working safely in a confined space, which is covered by the standard AS 2865 Confined spaces.

This document outlines many of the risks an hazards with entering or accessing confined spaces, such as manholes. Watercare can been seen here performing one of the tasks, by using an MSA Altair 4XR portable gas detector as the manhole is "cracked", to determine the prescence of potentially hazardous gases.
Watercare has been undertaking significant waste water testing in the greater Auckland region during August 2021, with the recent outbreak of the Delta strain of the Covid-19 virus.

Manholes are typically Oxygen-deficient atmospheres, and working in and around them can cause unconsciousness, brain damage and death. Oxygen deficiency can be caused by factors such as consumption by bacteria or displacement by other gases. These gases include Methane, nitrogen and Carbon Monoxide, all that can be present in waste water networks in specific volumes.
To learn more about the processes, steps and risks associated with entering wastewater networks, click the following links; https://www.worksafe.govt.nz/topic-and-industry/planning-entry-and-working-safely-in-a-confined-space/ https://wslpwstoreprd.blob.core.windows.net/kentico-media-libraries-prod/watercarepublicweb/media/watercare-media-library/forms/manhole_lid_opening_procedures.pdf