A New Zealand off-shore oil rig - like a spilling US counterpart - lacks a remote failsafe shutoff switch that may have prevented an environmental catasrophe in the Gulf of Mexico.

But the Australian oil company operating the rig, in the Tui oil field off the coast of Taranaki, says such a device is unnecessary here because oil is drilled at a much shallower depth, making it accessible for emergency action.

The US well spewing crude oil into the Gulf, threatening the ecology of vast stretches of the Gulf, did not have an "acoustic switch" to remotely choke off oil flows in an emergency, reports the Wall Street Journal.

The device costs about US$500,000 and is mandated in some oil-producing countries, such as Norway and Brazil.

It lets oil rig operators control seafloor shutoff valves from a life raft, even after an emergency evacuation. Other switches to activate shutoff valves are on the rig itself.

"We don't deem it necessary," said Dennis Washer, the New Zealand general manager of AWE, which operates the Tui oil field.

"We don't have an acoustic control - but that's not a bad thing at all.

"Some rigs have them, some don't. They're not seen to be an addition to the base case. It's very much a means of last resort."

Mr Washer said it was "quite wrong" to draw parallels between drilling in New Zealand and the Gulf of Mexico.

"The Gulf of Mexico operations are in extremely deep water ... as opposed to the more standard and benign operations that we have in New Zealand, which is far shallower."

Mr Washer said he was confident that equipment and processes at the Tui field were world-class, well maintained and could cope with an emergency.

Shutoff controls are typically at the base of a rig where drilling takes place, underwater where the shutoff valves' hydraulics systems are, and above water in the superintendant's office.

In the Gulf of Mexico, engineers were yesterday trying to attach an emergency shut-off valve to one of the well's three leaks.

BP, which had leased the rig, was preparing to drop a huge 4-storey, 70-tonne containment dome on top of the heaviest leak to keep oil from spreading.

The Wall Street Journal said that it was unclear whether acoustic switches had ever been tested in a real-world accident, saying they were intended as a last resort.

Some major oil companies, including Royal Dutch Shell PLC and France's Total SA, sometimes use the device even where regulators do not call for it, the newspaper said.

Maritime New Zealand environmental analyst Alison Lane said regulations in New Zealand required oil companies to prove they had adequate emergency procedures in place during annual audits, but the rules did not specify what systems were necessary.