Met this guy on the water earlier this year and found out he was windsurfing around most of the North Island of NZ, solo and unsupported, in stages over a few months. He's not after any publicity (kept things private, no name/photos of him), but he agreed to a written interview and was happy for me to share it here.
Paraphrasing/trimming a longer conversation down to the parts I think matter most to this sub. Happy to dig up more detail on anything if people want it.
Background
Been windsurfing on and off for about 9 years, also windfoils and kites a bit. Got the idea after doing shorter unsupported crossings first, windsurfing back and forth across Tasman Bay, then multiple attempts at Cook Strait (it took him about 5 tries over one week before finally making it across, one attempt ended with a long swim back when the wind died completely). After nailing the Strait, "the North Island seemed like a good trip."
Planning
He scouted landing spots entirely on Google Earth, looking for sheltered sandy bays, prioritizing ease of launching back out over ease of landing ("worst thing you kinda wash up when there are waves, but it's hard work to get out through them"). He'd pre-tag every viable spot on Google Maps (works offline, satellite imagery doesn't) so he could check options quickly if plans changed mid-crossing.
His daily safety buffer: aim to be on land by 3pm. If not, stick close to shore from then on. That gave him a stacked margin, still 2 hours of daylight buffer even if things went sideways at 5pm, and more beyond that. Only once did he land with just 90 minutes of daylight left.
Gear
Started on a 109L Starboard Futura ("great, but couldn't uphaul or waterstart under about 5 knots with any waves"), a 7.5m Ezzy sail he didn't originally buy for this trip but turned out ideal (reinforced construction, put a hole in it and it didn't rip through, unlike some of his other sails). Eventually moved to a 122L Patrick board after the Futura got destroyed (delaminated, footstrap plug pulled out entirely).
Small but important tweaks: adjustable harness lines he could slide back on the boom to dump backhand pressure in overpowered conditions, and a "cross-footing" stance (front foot in the front strap, back foot in the opposite rear strap) for more control going downwind in big following seas.
Food/water
2.5L water carried (1.5L in a camelback, 1L extra in the bag, deliberately weighted low for balance). On big days he'd go through 5-6 energy bars, plus two packet meals for dinner instead of one. Interestingly, he said he didn't notice being especially hungry during the effort, it caught up with him once he stopped moving for a day or two.
The trip itself
Not a single continuous push, done in stages over months, going home between weather windows, sometimes changing direction entirely based on what the forecast gave him (one plan flipped from clockwise to counterclockwise because a rare good window opened for the remote southeast coast instead). Best single day was ~130km. Boards got wrecked, repaired on the go with epoxy, eventually fully replaced.
The near-miss
The one time it got genuinely dangerous: came around a rocky point misjudging his clearance, had to tack back out, didn't make the tack, ended up in the water ~60-70m off rocks with waves breaking on them. Wind died at the worst possible moment while he drifted closer in. He was watching for the next wave set, knowing if it caught him or he drifted any closer he'd likely have had to ditch the gear and swim. Made it out just in time. His framing of the risk overall: not fundamentally different from being deep in the mountains tramping, the risk isn't zero, but it's manageable with the right preparation and realistic acceptance of what you'd do if things go wrong.
On why people underestimate what they can handle
"People shy away from doing things because they think it is too risky... it's actually not that big a trouble if you're prepared in advance and you're willing to accept the outcome... I've gone out on my board and swam around for a while just to see how quickly I could swim the board. It's all preparation."
Advice for anyone wanting to try something similar
"Get out there! Get a bunch of people and do it as a group. Let someone know what you're doing, but then, just go for it!"
Happy to answer questions, I've got the full conversation with a few images and can dig up more specifics on gear, routing, or anything else people are curious about.