Heavy rainfall in the North Island this year has made for a challenging truffle season for Bay of Plenty producers.

Growers have been out since early June harvesting the fungi, but wet weather has hampered progress, causing many truffles to rot.

There were more than 300 truffle farms, or truffières, in New Zealand - most supplying the domestic market.

Maureen and Colin Binns of Te Puke Truffles just wrapped up this year's harvest in Paengaroa.

Maureen Binns says extreme rainfall has had a big impact on the size of the black truffles.

"The adverse weather events have really affected us quite dramatically in terms of truffles - production, size.

"As it's turned out, we've been able to find truffles on each of our hunts and we've been able to supply our two partner restaurants here locally, Sugo and The Trading Post.

"But size-wise, it's been the worst season ever."

Another grower, Marie Dennis - who grows black Périgord truffles at Tikitere, north of Rotorua - is still busy harvesting.

The ongoing rain has also caused big issues for her crop, with high levels of rot - but she said free-draining soil has been key to retaining good truffles this year.

"I'm picking a reasonable amount out every week," Dennis says.

"We're very lucky because where we are, we've got a nice layer of topsoil on the top and then we've got a very good base layer of pumice, so our property is very free draining, which means a lot of that water has been able to be taken off the truffière.

"But in saying that, there's been a raging stream or blimmin' near a river going through our property in a couple of those big downfalls. We've never seen it like that before, but luckily all of that actually drained away pretty easily."

Despite the wet weather, Dennis remains upbeat about this season as demand from restaurants and new consumers was still strong.

Producers in Western Australia are reporting another bumper season with increased demand for the lucrative fungi - partly due to a bad season in France last year, due to drought.

-Monique Steel/RNZ.