Talau, who was plucked from reserve grade to play standoff in last month's trans-Tasman test, was already due to join St Helens next season.
However, the Bulldogs received a call from St Helens last week after a high injury toll forced them to miss the annual Middlesex rugby sevens tournament.
Bulldogs chairman George Peponis said Talau would leave immediately although they had the option of recalling him if the club experienced similar problems in the run into the National Rugby League (NRL) grand final.
The retirement of former Bulldogs captain Darren Britt after suffering a fractured eye socket last weekend freed up an import spot on the St Helens' roster.
Since being injured earlier this season Talau has been unable to force his way back into first grade, with international team-mate Nigel Vagana and rookie Ben Harris forming an impressive centres combination.
Peponis said Talau left with the club's best wishes.
"It's an unusual decision but we don't want to stand in the way of his career. Willie's given the Bulldogs good service over the years and he's got an opportunity to earn good money over there."Like