Aeromarine Industries Ltd operations manager Simon Robb said it was "unfortunate" the incident had happened at the same time as the closure of Alpine Aqualand's fast-speed hydroslide.
Up to 69 customers have received "minor injuries" on the slide since the $18 million aquatic centre opened on May 31.
Operators Lakes Leisure closed the fast slide last week.
Of the 69 injuries, 51 were caused by users breaking the safety rules, such as by standing up in the slide. Up to six people were concussed.
The hydroslide rules are displayed on illustrated signs at the base of the stairs and at the top between the two slide entrances.
"Failure to observe these rules may result in injury," the signs read.
The 45-year-old female New Plymouth lifeguard was reported to have been admitted to Taranaki Base Hospital's intensive care unit with concussion and a cut to her head.
She had apparently lost some teeth in the incident.
Mr Robb said the New Plymouth slide was a 16m high prototype.He said he believed it to be the largest in New Zealand.
"The reason it wasn't open was for testing.
"It's unfortunate that's what's happened, but that's why we have tests and I myself have been down it many times without incident."
He said the New Plymouth slide would be recalibrated and declined to comment further.
Department of Labour spokeswoman Debbie Hannan confirmed the government agency was investigating the New Plymouth incident.
The department had not been formally advised of any injuries at Alpine Aqualand, "but given the information in the public domain, we have someone discussing these reports with Lakes Leisure management", she said.
Lakes Leisure aquatics general manager Cam Sheppard said he had no concerns about Aeromarine Industries as the slides were very different with different issues.
A Department of Labour health and safety inspector had visited Alpine Aqualand yesterday to follow up an anonymous complaint from a member of the public who had slipped and fallen on the floor surface.
"We have every confidence the floor quality is of the highest standard and completely safe. He was aware and interested in the slide but didn't take it further."
Mr Sheppard said the Alpine Aqualand fast-speed hydroslide, about 7.5m high, was standard and "tame" in comparison with the slide in New Plymouth's Todd Energy Aquatic Centre.
A weir would be added to the top of the fast-speed slide to cause the water to flow at a slower rate, he said.
"From there it's a matter of the [Lakes Leisure] board being comfortable the changes meet their needs," Mr Sheppard said.
Modification and testings were to be carried out yesterday and today.
"Between better management and minor calibrations, we should be able to decrease incidents markedly."
Aeromarine Industries Ltd completed hydroslide projects in Timaru, Napier, Whakatane, Taupo, Palmerston North, Queenstown and New Plymouth, with one to be installed in Hawera.
Lakes Leisure sold more than 14,000 slide passes since Alpine Aqualand opened on May 31, and customers took on average five slides each, he said.
