"I could talk about gardening forever," she fizzes at one point over coffee and cake.
Since she first dug, six summers ago, into the black soil of the then-new lifestyle block she and husband Todd bought near Waiuku, O'Neil's garden has grown and grown, not just vegetables but in size - it now has "about" 22 beds; she says "about" because it apparently depends on how you count them. Her garden seems to have grown, too, from a hobby to an obsession to something that she now shares, through a blog and now through her book, with other gardeners and want-to-be-gardeners around the country.
Any given day of the year O'Neil will be planning, planting, weeding, digging, clearing, hoeing and harvesting from her garden, with the results - the fruit and vege of her labour - either going straight into the pot for that night's dinner, or into a variety of preserving schemes (from freezing to bottling) or they might become part of her latest, slightly mad, experiments in the kitchen.
"I am going through a phase at the moment of trying to hide vegetables in chocolate cake and muffins," she tells me, moments after I arrive, and she points to a plate of muffins.
"Those have beetroot in them. Last week it was zucchini chocolate cake. They're experimental but they taste good."
This, you see, is not just a garden, but a philosophy. As she writes in The Good Life, "Gardening isn't really a hobby ... it's not just about growing things to try them out ... it's not just about exercise and getting out in the sun, it's not about teaching the kids where their food comes from ... it's not really about giving my family food that I know where it came from ... and about being organic ... For me, gardening is about having home-grown produce all year round."
It's about the quality of the food too, she tells me. "A freshly picked capsicum, when you slice a knife through it, it's crunchier than a fresh granny smith. The sound it makes putting your knife through it - a supermarket capsicum will never make that sound. It is just so fresh. You know the best way to have asparagus? The best way is that you get your pot boiling first, you get your steamer on and when it is at temperature and ready to go, then you harvest - and you run, you cut your asparagus and you run back to the kitchen! Fresh is the thing ..."
All of which - the freshness, the enthusiasm - is a long way from her first garden: two pots in her tiny backyard in the dusty big smoke.
A sign, it turned out, was the sign to change her life and to get the hell out of town. It happened six years ago when a billboard advertising retail space went up near the O'Neils' first home in Mt Wellington's Lunn Avenue, a sign boasting that the passing traffic numbered around 20,000 cars a day. The couple finally decided that it really was time to move, to get out of the city.
As she writes in The Good Life, "our first home was just that, a first home, the bottom rung on the property ladder ... The soil was dusty and full of volcanic scoria, I don't know if it was fertile or not as there wasn't really any place suitable for a complete novice to create a satisfactory garden bed, I had to content myself with a pot for a half a dozen strawberries and one for tomatoes and peas to take turns".
What O'Neil had, however, was a well-thumbed Yates Gardening Guide.
"It was well-thumbed because it was a dream to have a proper garden," O'Neil says. "I didn't do it, I just dreamed it. We didn't have the space. I think Tom would've killed me if I'd turned what little lawn we had into a vege garden because the lawn wasn't very big."
Gardening was in the family. Her grandfather was a market gardener - she grew up seeing pictures of his fields of cabbages and carrots near Ohakune - and her mum always had a vegetable garden when she was a kid. "But it was her thing and she did it and provided the food for us. We didn't take any notice of it as kids."
But with no land and two young children of her own, O'Neil's dream seemed unlikely to happened on Lunn Ave. And then she was diagnosed with multiple sclerosis, an autoimmune disease that affects the brain and spinal cord.
"I had my first attack two weeks before Joey was born. So there I was with a toddler and the baby. Tom had recently put the car on the market with a 'for sale' sign on it and he'd had it professionally valeted and it was out there on the road for a week. When you ran your finger along it, there was soot all over it. I thought 'this can't be good for us'. Then the sign went up out the front advertising retail space saying that 20,000-plus cars drove past every day.
"We needed to do something different. It took a health scare and the other city things to think 'we need to move'. But [the city] was all we knew. We were giving up everything. We knew no one, we knew nothing."
After carefully accessing the rural areas north, south and west of Auckland, the couple and their young boys - Tim, who turned 9 two weeks ago and Joey who is 7 - settled for the 1.2ha property a few kilometres outside of Waiuku in the summer of 2007.
"The land had to be flat, it need to garden-able. I didn't want to buy a property on a slope where I couldn't put a garden in. I thought 'we can move to the country - oh, and I can have a garden!' It was that kind of a thing. It wasn't 'we're going to move to the country so we can have a garden', it was like we can move to the country and have a better life - and have a garden!" With the ill health you start to look for ways to improve your life. To have fresh produce is just so much better for you than buying it."
Her garden, that first summer, was just amazing, she says "We planted tomatoes, pepper, beans ... We went bananas the first year." But the signs were there that her ambition was quickly outstripping her experience. For example, the lettuce patch: "When you've allocated an area for your lettuces don't tip the whole packet of seeds into that, because there's about 1000 seeds in the packet! So seriously don't do that. It was good for a while and then they all grew into each other!"
The first winter her beginner's luck appeared to wash away, along with her carrots. Heavy rain revealed that the property was on reclaimed swamp - it was drained in the 19th century to provide summer grazing - so O'Neil insisted Todd (who she dubbed "Hubby the Un-Gardener" in her blog and book) dig holes all over the garden so they could bucket the water out. "But the more we bailed out the water the quicker the holes filled up," she writes in The Good Life. "When I saw some of my carrots float away, we decided to abandon this garden and build raised beds using material from the fences on our newly acquired property."
O'Neil says her gardening, other than what she'd gleaned through the Yates Garden Guide and YouTube, has been - and mostly still is - trial and error. "[My philosophy is] 'let's give this a whirl, let's see if it will work'. But I can grow a carrot now. I am really proud of my carrots this season, they look perfect."
Over the last few years, her gardening has not only fed her family, it's improved her health. "Vitamin D is really good for people with MS and I am out in the sun everyday.
And physical activity helps self-esteem because, when you're stuck in the city in a dark little house with very little backyard space and two small children, it is very easy to become depressed. I'd almost become a shut-in because I didn't have any self-esteem or confidence, it was completely gone, and I let the disease rule me."
However, it was her decision to start writing about her garden that has taken O'Neil in an entirely unexpected direction. During the summer of 2010-11 she entered a Yates-run blogging contest, with readers voting on a winner each season. "You blog over three months, a 100 days or so. I had been gardening for a couple of years and figured 'I'll give this a whirl' and found I enjoyed the writing - I'd never done any formal writing before."
She didn't win that summer, but won the following season writing about her winter garden. She used her $1000 prize to build herself a shed. Meanwhile all those words - her blogs totalled some 130,000 of them - sat on Yates' website unloved.
"I was like 'don't bother them', but Tom rang Yates up and said 'we can't just let those words sit there dead on the website'. We were going to see if we could get them back for us because, by this stage, the blogging competitions had finished, though I set up a wordpress blog and started doing that and still do. So Tom rang Yates and said 'we're looking to self-publish something or drip feeding the words back on to my blog ...' and they said 'if our publishers agree, we'll sponsor a book'. Tom's jaw fell to the floor. Tom emailed [publisher] HarperCollins ... with some samples of my blogs. HarperCollins phoned back in two hours. The whole thing since then has completely and utterly gobsmacked me. They gave me 88 days to sort the book out."
The end result is a chatty, inspiring four seasons in O'Neil's garden, a mix of success and failure, fun and hardwork with plenty of photos she took herself. There are recipes too, but only after her brother helped out.
"The recipes in here ... well, I am no Annabel Langbein. I am not a fantastic cook, but they work. My brother is a chef on luxury yachts in the Mediterranean, so I got him to run through my recipes and there is a recipe in here for spring onion soup. He comes back to me and says 'Sarah, this is not how you make soup!' I am like 'well it worked and it was really nice!' So he reluctantly adjusted all my recipes to make them suitable for the public."
Ask O'Neil who the audience for her book might be and she bellows: "Everybody!" However, she concedes, not everybody loves gardening. "I was shocked [by people not like gardening] because I just assumed that everybody secretly wanted to garden but didn't have the time, the space or the money, because it's so good for you. But some people actually don't like it."
Her book, she hopes, might change minds. "It's about real gardening, this book. It's not picture-perfect magazine gardening. There is a photo in here of some rotting zucchini. It's real. It makes gardening more accessible. But gardening is hard work, I won't lie. But there is so much reward from gardening. I think everyone should give it a go, even if you just drill some holes in a bucket and get a $3 punnet and bung them in the bucket. It'll cost you less than $5 and you'll havefresh broccoli or whatever - and the taste of it is something like you wouldn't believe."
• The Good Life by Sarah O'Neil (HarperCollins $39.99) is out now.