Todd Energy is planning to drill three wells in the next 18 months to check whether a little-known onshore Taranaki gas field could become a major producer.

In the past, gas in the Mangahewa field, 18km southeast of New Plymouth, was considered to be held tight and difficult to extract.

But the Mangahewa-3 well, which was drilled in 2007 and brought into production last year using modern techniques, led to a reappraisal of the field's potential.

"The work we've done ended up in us drilling Mangahewa-3, which was certainly a very successful well," Todd Energy managing director Richard Tweedie said.

If the three planned wells produced similar results to Mangahewa-3 "then we're on to something pretty significant and we're looking at something that is potentially a Pohokura size".

"It would require a lot more infill drilling, so we'd embark then on a major drilling programme," Tweedie said.

Offshore from Taranaki, Pohokura is at present the country's largest gas field.

Tweedie said if Mangahewa turned out to be as good as was hoped, it could end up having more than 1000 petajoules of reserves.

At current use rates, that could push New Zealand's P50 gas reserves - those estimated to have a better than 50 per cent chance of being technically and economically productive - out to 2030 or beyond.

Without Mangahewa, he said, this country has enough P50 reserves to take it past 2022 at current demand levels.Tweedie's estimate is more optimistic than that of some others in the industry who see known reserves having a shorter lifespan.

An exploration well was first drilled in Mangahewa in 1961 by a Shell-BP-Todd joint venture.

Traces of gas were found but the well was plugged and abandoned.

Tweedie said that at the time everyone believed the gas was tight, meaning the gas could not be satisfactorily produced at commercial rates.

Fletcher Challenge Energy drilled a second well in the mid-1990s and permanent production started in September 2001.

That year, Shell bought Fletcher Challenge Energy and, to gain Commerce Commission clearance for the purchase, the company sold Mangahewa and nearby McKee to Todd.