Stormwater Drain blocked (QLD)

At the rear of my duplex is a fence and along the boundary of the fence, (forming part of the fence) two very large trees.  The roots of the trees have gone into the stormwater drain running along this fence which also services 3 other duplexes.  The Strata Manager has said that the repair of the stormwater drain is my responsibility including the removal of the two trees on the boundary and the repair of the fence.  Please note I did not plant the trees!  They were already there when I purchased.  The cost to me will be $2K dollars.  I put it to them that if the stormwater is my responsibility because the blockage is on my side from the roots (and probably on the other side too but that has not been checked yet) then I should be able to have the plumber redirect the drain on my property direct to the council stormwater drain thereby leaving the problem with the other owners having fixed my side.  Who is responsible for stormwater drains please?  Thanks     I