National is promising to lend farmers up to $300 million to help them store more water, as it targets the rural vote in this year's election.

The party said it wanted to make it easier for farmers to collect rainwater when there was plenty of it, then use it to grow crops and feed livestock when conditions turned dry.

National leader Christopher Luxon and agriculture spokesperson Todd McClay unveiled the campaign policy at Feilding's annual Rural Day event.

McClay said the country captured and used just two percent of its annual rainfall.

"New Zealand receives more rainfall per person than almost any developed nation on earth, yet too many of our regions still face real water insecurity, because it often arrives in the wrong places and in the wrong seasons."

He said another 400,000 hectares of farmland could be irrigated if more storage schemes were built.

"When farmers and growers have reliable water, they produce more, take on more staff and more money flows into the local town, which means more jobs and stronger exports."

The $300 million would be offered as low-cost loans to paid back within ten years.

As a result, National said the loans would not eat into the government's budget for new capital spending, though there would be an annual write-down cost of $16.3 million, reflecting the more generous terms than usual.

National said that cost would be managed through the existing Primary Sector Growth Fund.

Five regions would get first consideration: Northland, Tairāwhiti, Hawke's Bay, Wairarapa and Canterbury.

A national water storage register would be set up to work out which schemes should get priority and to publish a wider water infrastructure pipeline alongside farmers, growers, councils and iwi.

The party also wanted to introduce nationwide standards for irrigation and lower-risk water storage. That would allow farmers to build some smaller on-farm storage facilities without applying for resource consent and make it easier to get approval for irrigation systems.

As part of its wider announcement, National also promised a $10 million fund to help produce more fertiliser in New Zealand, reducing the country's reliance on overseas suppliers.

It commited to strike agreements with other countries to keep supplies flowing if global trade was disrupted, using New Zealand's existing deal with Singapore on essential supplies as a model.

Associate agriculture spokesperson Mike Butterick said fertiliser was just as important to the economy as fuel and electricity.

"Without reliable access, our farming sector could not sustain anything close to its current production base."