I have not been there personally, so I have to rely on others.
This blog report has references to actual locations he used..
https://waynemerry.wordpress.com/2013/01/02/the-bluffmt-mcdonald-trip-new-years-201213/Relevant snippet:
"Our first water stop was from the headwaters of the Jamieson River, just underneath Picture Point at 634832."
{I believe this is a different arm to the one with the pipe and they just went down they slope till they found water in the gully.}
Another blog gives very detailed descriptions - but no grid references.
https://timbo.id.au/The%20Howqua%20River%20to%20Mt%20Howitt%20circuitRelevant snippet - describing the descent of King Billy 1:
"If you like you can follow the Alpine Track and cross King Billy One and King Billy Two. Or you can just take it easy and follow Bluff Track road until you get to the turn off for Mt Magdala (which is posted with an Alpine Track marker). There are camp sites at the turn off.
But! If you follow the old and faint four wheel drive track that heads off from the camp site/clearing beside Bluff Track in a vaguely eastern direction (it quickly becomes a foot track and then fades away completely) you will find more camp sites. You will also find seeps and if you go far enough, a little spring. There are a whole series of lovely little, verdantly green clearings that have been lovingly “mown” by the local animals and they make wonderful, sheltered and quiet tent sites."
I believe this is the campsite the school kids were aiming for. They called it the King Billy campsite and I believe it is right at the track junction of the Bluff track with the AAWT as it heads for Magdala.
But looking at this description again - I may have got my directions wrong. This indicates that this spring is actually in the headwaters of King Billy Creek - not the Jamieson.
But it is not impossible that water could be found descending on either side - so I am now confused as to where exactly the pipe might be.
Perhaps you could ask Timbertop School about it - as they are regularly going through that area?
Based on this report though - I would be heading east from the track junction.