It's shaping up to be a dry October, farmers have already started irrigating in mid-Canterbury and NIWA expect similar conditions to persist until Christmas, but no one is crying "drought" just yet.

Chris Brandolino, principal scientist at the weather agency, said the dry weather had been brought on by persistent high air pressure systems.

There was also a possibility of the El Nino weather pattern developing, although Brandolino said he expected it to be weak if it does eventuate.

"Above average rainfall looks unlikely," Brandolino said, adding soil moisture levels were already low across the northern South Island, parts of the Bay of Plenty and Waikato.

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"The tea leaves are for normal or below normal for the rest of the year," Brandolino said.

"It is certainly worth watching because it is dry in some areas - drier than this time of year should be in terms of soil moisture levels," he said.

El Nino, if it arrives, did not automatically mean wet conditions in the west and dry weather in the east would eventuate, he said.

Chris Allen, who has a sheep and beef farm in mid-Canterbury, said the dry weather was already having an impact.

"It's pretty dry and that's starting to show up in the shallow aquifers and some of the river flow levels, so irrigation over the last week has had to fire up," he said.

But no one is raising the spectre of drought just yet, said Allen, who is Federated Farmers' spokesman for water and the environment.

"Farmers are thinking about it," Allen said. "It will take two or three weeks for them to start making decisions," he said.