There's a computer in your boot (below the parcel shelf on the drivers side) which controls the HICAS (rear wheel steering) and the Power Steering...
This often gets flakey and dies (mine only does it sometimes if I restart the car after a long drive and it's quite warm).
The computer controls Power Steering stiffness... It is meant to be quite "light" at very low speeds to make parking easier, and stiffens up a bit once you get moving, and then goes fairly stiff at 80km/h+ to increase stability. My computer has a loose solder joint or something, so sometimes stays "stiff", which makes it feel like power steering has failed. But really it just thinks you're going faster than you are, so it stays hard.
If it happens all the time, find a friend with another R33 GTST which is the same series as yours, and swap the computers over (few bolts and a plug, really easy and quick to do) to see if it helps. If it does, grab one for $40 from someone who's wrecking a car in the For Sale section