In 1977 Mr Sweet, a teacher and architect, built an experimental house on the property.
A teacher lived in it for a while and, when he left in the early 1990s, Mr Sweet restored that house too, and added it to the retreat business.
Then, in the mid-90s, a back injury forced him to lease the "basic sheep and beef" farm rather than running its remaining 240ha.
These days the Sweets have enough beds to sleep 22 people. But they don't usually have that many all at once - a regular French group of 18 is usually their biggest. This year they hosted people for 254 bed nights in the three months from January to March - their best total in four years.
Both their extra houses have kitchens but most guests have at least one meal at couple's Homestead. Mrs Sweet makes European recipes using good New Zealand produce - you're more likely to be served a sophisticated rack of lamb than a plain roast dinner. Her three-course dinner, in a dining room full of history and homemade native timber furniture, costs $45.
"I would rather eat here than most restaurants," says her husband.
Guests are mainly from Europe, Australia, the US and New Zealand. The Sweets have links with several tour organisations, helped by Mrs Sweet's fluent French, German and Italian.
They meet "lovely people", including many French from New Caledonia and Tahiti, on secondment from the military.
Most stay only one or two nights. The cost is similar to a motel - $100 to $150 for each couple, with an extra $40 to $50 for each additional person.
Guests can wander the park-like grounds, with their mature native trees and views of the Ruahine Range.
Activities include tennis or croquet, swimming in the Mangawharariki River, fishing with guide Jim Rainey or walking.
Mr Sweet usually takes them to see the horses daughter-in-law Leonie breeds on the property, because the foals are charming. He used to take farm tours in his 1953 Landrover - but will not be doing that under new health and safety laws.
Mairenui Rural Retreat has its own website and guests can tap into wireless internet, thanks to a link bounced from son Matt's house nearby. Outdoors there's not even cellphone coverage - a remoteness some guests enjoy.
Hosting people is a full-time job for the Sweets in season - with cooking, cleaning and guiding.
"This business is quite all-encompassing during the summer months. We have no social life, and we can't leave," Mr Sweet said.
Fortunately, son Matt and his wife live on the property and help out. Matt is a chef who works at Flat Hills Cafe and his wife Leonie takes care of one of the houses as well as breeding horses for sale.
