All Black joins Clermont
Clermont have been looking for a replacement for first choice flyhalf Camille Lopez since the France international broke his ankle against Northampton last month, leaving him facing a four-month spell on the sidelines.
"The last few weeks our problems at fly-half and scrum-half have been building and this signing comes as an oxygen boost," said Clermont's head coach Franck Azema.
The 34-year-old McAlister, who has 30 All Blacks caps, only joined Toulon from Toulouse, where he spent six years, in the off-season.
He found himself third choice fly-half behind French internationals Anthony Belleau and Francois Trinh-Duc, though.
As well as Lopez, Clermont lost Argentine flyhalf Patricio Fernandez for two months due to a dislocated kneecap suffered against Lyon on Saturday.
The champions have an injury crisis at the moment with new Tongan prop Loni Uhila the latest to suffer an injury.
Fiji-born France international wing Noa Nakaitaci (ruptured knee ligament), France lock Arthur Iturria (shoulder), captain Damien
Chouly (herniated disc), Scotland scrum-half Greig Laidlaw (broken leg) and Georgian prop Davit Zirakashvili (chest) are also in the treatment room.
And Clermont had the unwanted news confirmed this week that Nakaitaci has signed a contract to join Lyon next season.
Lyon, who have surprised everyone this season by topping the table for several weeks, until their defeat to Clermont on Saturday, announced a spate of signings for next season, including another France international, fly-half Jean-Marc Doussain from Toulouse.
Clermont will also lose prop Raphael Chaume, who made his France debut 10 days ago against New Zealand, to Lyon, who are taking Oyonnax back row forward Patrick Sobela too.
New Zealand centre Rudy Wolf, 33, South African prop Albertus Bukle, 34, Argentine front row forward Francisco Gomez-Kodela, Australian back row forward Liam Gill, French wing Alexis Palisson and club captain Julien Puricelli have all signed contract extensions for Lyon.
Meanwhile, Fijian wing Alipate Ratini, who has already been sacked by Grenoble, Stade Francais and Perpignan over the last couple of years over alcohol and misbehaviour related incidents, has resurfaced at second division Beziers, 11-time French champions who have fallen on harder times since the 1980s.
"Alipate is a player with exceptional physical potential, I think he can bring us something extra in terms of ability to break the gainline and score tries," said Beziers coach David Aucagne.
Despite seven tries in 11 appearances for Perpignan in the Pro D2 this season, Ratini was sacked over behaviour that was "bad for his health", the Catalans said.