Don't let labels get in your way.
Girth will be calculated from the two smallest dimensions: 380 and 280 (millimetres I presume). This gives a girth of 132cm which is within Australia Post limits - but the length of 122cm is over the 105 cm length limit.
For international shipments, the Australia Post postage calculator website doesn't have any provision for a length limit. It just mentions the same limit on girth. However, if you look further down the AusPost rabbit hole, the 105cm length limit still applies. Even without that limit, Australia Post options start on the wrong side of $100.
So, those shipping figures can't have come from Australia Post.