Dr Mike Shepherd, a Starship emergency department specialist, said of the bill: "I think it is reckless ... They are proposing an unscientific experiment."
The hospital says the 1987 act has been highly successful in preventing harm to children.
"We estimate that since its introduction, at least 200 New Zealand toddlers' lives have been saved and at least as many children have avoided permanent severe brain injury."
Most children who die in home swimming pools, the hospital says, are found fully clothed, suggesting they were playing when they fell in, and investigations have shown how quickly and silently they drown.
"This bill removes the current mandatory requirement for pools to be fenced, replacing this with potentially multiple, duplicated and untested measures permitted within acceptable solutions, alternative solutions, determinations and waivers.
"One clause of the proposed Acceptable Standard ... reduces the existing safety of pool fence gates. This section does not require gates to be self-latching, as currently required in the ... act."
Instead, gates must be "not able to be readily opened by children", although they will still have to open outwards and swing shut automatically.
"Auckland council currently inspect pools three-yearly and report a 50 per cent failure rate on first inspection. Most failures are for gates and are fixed promptly at the time of inspection. This suggests three-yearly inspections are both critical for safety and are highly cost-effective."
Dr Shepherd said that instead of watering down the law, MPs should strengthen it by measures such as requiring pools to have "four-sided" isolation fences rather than allowing direct access to a pool enclosure through a child-restricted house door.
Pool fence laws
• Most swimming pools, including spa pools, must have a 1.2m-high fence with a self-closing, self-latching gate.
Inspections
• The councils, including Auckland - where 60 per cent of pools are sited - inspect the fences every three years.
• Those covering another 20 per cent inspect the fences at varying periods.
• The remaining 20 per cent are never inspected.
Proposed changes include
• Five-yearly inspections everywhere.
• Pool suppliers must tell buyers of their obligations to restrict children's access.
• Spa pools with child-resistant covers will be exempt.