Hala'ufia joined the Scarlets during the summer ahead of the 2014-15 season and is yet to make an appearance in the region's colours.
The Tongan born number 8 left London Irish at the end of 2013-14 after six seasons with the Aviva Premiership club. The 35 year old, who also represented Harlequins in the Premiership, has 24 caps and five tries for Tonga.
Scarlets Head Coach Wayne Pivac said; "London Welsh are in a position to give him some game time. He'd requested to be loaned and it has been agreed between the three parties."
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