A couple still have no idea what led to the presence of what was thought to be methane gas around their house which put one of them in hospital with poisoning.

The incident happened last Thursday afternoon on the corner of Mosston and Fox Rds in Whanganui.

The husband didn't want to them to be identified but said his wife had been outside and when she came in told him she could smell gas.

"So I poked my head out the door and had a sniff and said to her it smelt like gas," he said.

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"Then she's walked down the driveway to shut the gate.

"When she got back in she said, 'I feel dizzy, I don't feel very well'. And that's when she laid on the couch and within five to 10 minutes she was having seizures from it."

At that point the man was already talking to police and he told them he needed an ambulance.

"So they sent the fire brigades and the ambulance around ... the fire brigade told the ambulance staff that it was methane.

"The ambulance got my wife into a chair, wheeled her out and took her out to the hospital."

She was improving but advice from hospital staff at the time was further evidence of a gas being present.

"They had her there on observation ... about four hours later she was well enough to come home," the husband said.

"She had all the symptoms ... her lips were swollen. Her legs - she said she felt like she had compression on her legs. She had all the symptoms of gas poisoning, according to the hospital."

Fire and Emergency reported picking up methane gas coming from a septic tank at one of four properties surveyed at the time.

The man said his wife had noticed a similar smell a fortnight earlier and he was frustrated he hadn't had any explanation for the gas.

"I'm disappointed I haven't had anyone come back again and see us or give us any information or anything.

"That's what I'm brassed off about. I had to ring the gas department myself ... they came out and asked me a few questions and walked up and down the side of the road, said they can't find anything and then went away."

He was now worried the gas will come back.

"It's obviously going to happen again and if someone outside lights a smoke or something it's going to go bang.

"About an hour earlier on I had my grandson around here and my daughter. He's only 20 months old. If he got asphyxiated he mightn't handle it as well.

"It's not something I want to leave until something happens."

GasNet manages the pipes around Mosston and Fox Rds and sent a technician to investigate, but that was the next day.

"Our technician attended and surveyed the gas pipes in Mosston Rd and Fox Rd in the vicinity of the area," said Wayne Armishaw the engineering manager at GasNet.

"We used our gas detection equipment but we didn't detect any gas at all."

Armishaw said his company could only check its equipment and wouldn't know if it was coming from something else.

"If our gas mains were leaking it would still be there the following day. If it was some other source, I wouldn't know.

"We've got some very sensitive gas detectors and we walked over the area where our gas pipes are and found no sign of gas there."