The former Tauranga Boys' College student had left for Bristol and a second stint as Gloucestershire's overseas player for the first half of the county cricket season, having spent the whole of last season in Bristol. He is expected to arrive in time for the county's first home Twenty20 championship game against Glamorgan this weekend.
Gloucestershire re-signed Williamson after New Zealand Cricket and an unnamed sponsor made it possible for the cash-strapped county to bring in an international hired-hand.
Director of cricket and former New Zealand coach John Bracewell said: "The bottom line was that we had no money in our budget for an overseas player.
"We got in touch with New Zealand again and asked them how it had worked for them last season. I had read a lot in the media down there about how their young players had benefited from playing over here, as opposed to playing a shorter format of the game all over the world.
"I was not quite sure how John Buchanan [New Zealand coach] would go with it, because the regime has changed since last year. Fortunately for us, he was pretty keen on continuing with the arrangement. That means we are getting a cheap and almost free deal. We have a sponsor who has managed to fund it and it is costing the club hardly anything.
"Kane is a centrally contracted cricketer and, as such, New Zealand are paying most of his wages, so not only do we have an overseas player, we have one we did not expect to have because there was no money."
Bracewell said he believes Gloucestershire are getting a better player than the one who cut his teeth in county cricket a year ago.
"Kane has played in all forms of cricket for New Zealand this winter and will come back a more experienced player and one who is more adept at switching from one form to another. Previously, he was predominantly a test cricketer, who did not play much one-day cricket. That has changed and he is going to be a better all-round player than before."
Williamson will be available to Gloucestershire up until shortly before New Zealand tour West Indies in July and August. Bracewell was keen on Williamson's Bay of Plenty and Northern Knights teammate Daniel Flynn replacing him but Flynn's half-century in the last test against South Africa is likely to mean he'll go to the West Indies, too.