The regulations for wall height can vary depending on the council but generally in most cases you can build a retaining wall up to 1 meter high without council approval

It depends on the height and location of the wall. If the wall is under 1m in height and not retaining lateral pressure then timber sleepers can be used as uprights . Lateral basically means "moving towards" so if your land has a 30 degree slope north to south and you want to build a wall on the east side fence line because your property is lower than the neighbours , the lateral pressure is from the top north end to the bottom south end . The pressure from east to west is significantly lower. Remember that timber sleepers are prone to rotting over time so always coat them with a thick bitumen paint on any area that will be covered underground

Unless its a one or two sleeper high garden edge with no lateral pressure then No , never back fill a retaining wall with soil or dirt. The reason is Hydrostatic Pressure , when it rains all the water runs to the lowest point which is usually behind the retaining wall , when the soil gets wet its weight increase significantly and almost doubles the pressure the wall needs to hold back the second reason is once the water is in the soil it doesn't dry out and starts to rot away the sleepers and reduces the life of the wall . You should always backfill a wall with scoria or a 40mm aggregate with no fines , run a slotted ag drain on an angle approx 200mm off the ground level , this will catch the water and drain it away , you can top dress the scoria with a 150mm layer of top soil if you intend to have a garden or grass in the area being retained