being a vy v6 you cannot tighten the belt, the tensioner does that automatically. now from memory their is 2 main possibilities.
1. the power steering pump pulley is out of alignment (quite common) this causes the belt to squeak as it is rubbing sat an angle over the pullies.
2. you have a worn idler pulley. a great way of checking is to have a long screwdriver and you put it on the nut of the corresponding tensioner whilst the engine is running (making sure it is a stationary nut or else it could get messy

) and then listening on the other end of the screwdriver. you will be able to hear which idler is blown cause it will sound like a dry bearing/ screeching noise.