If the fence is a typical 1.8 metre compliant fence then you do not need any formal approval to remove/replace it. If the fence is a structure that would have required approval to build – that is, it is not a standard height etc – then you do need approval to remove/replace it.
Keep in mind that matters involving boundary fences sometimes tend to create an amount of angst between neighbours, so it’s a great idea to discuss any removal or replacement of fences with your neighbours prior to commencement.