There is no requirement to have a fence if you and your neighbour don’t want one but if you want a fence and your neighbour doesn’t you should get a quote for one to be built and discuss it with your neighbour. If you don’t reach an agreement you can give the neighbour a written notice specifying the fencing work proposed. If after serving the notice you and your neighbour still cannot agree either may ask the Local Court to make an order about the fencing work required. If a fence is to be built you and your neighbour, usually, though not always will have to share the costs.
Further, usually, you will both share equally the cost of repairs to any fence between your properties unless the fence was damaged because either of you was careless then the responsible party must pay for repairs.