Retaining wall at front boundary

The front of my block has a slope down to the nature strip. The road is then flat and across the road it drops down again. From the set back its about a 6ft drop.

Am I responsible for a retaining wall at this point if its rqd?
http://imgur.com/W8nkuYR

Short answer yes, There is a trend to land developers removing retaining walls from subdivisions why?
1. Reduce the bottom line costs and initial sale price
2. These costs are passed on to the land purchaser
3. these cost are shared by adjacent neighbours...
4. councils aren't interested in sharing these costs..goodluck
5. Retaining & Site works are dealt with as PSs by the builder..so hand over a deposit and they'll sort it out later..goodluck
6. Unfortunately, the average punter always pays a premium

Not if the lie of the land is identical to when the house was built and finally approved as a dwelling, and no other changes have been made that contribute accelerated water flows to the area.