Intellectual property protection
Firstly, under Australian law, software that you produce will generally be protected by copyright - provided, of course, that it is entirely your own work and you have not used any infringing material. Secondly, assuming that the web app also contains graphics, photos, videos, music, etc., copyright will subsist in those as well - again, provided that they are your own work. Copyright will generally vest with the author unless it has been assigned in writing.
Note, however, that copyright does not protect the idea; rather it protects the expression of ideas in a material form. Copyright will not stop a third party from producing a web app with the same or similar functionality to your own app. Please also note that if you intend to distribute your web app internationally, different laws may apply in relation to (for example) the subsistence of copyright and exceptions to infringement.
Software can also be patentable if it meets the parameters of a 'patentable invention' as defined in the Patents Act 1990(Cth).Depending on what the app actually does, it may be patentable, but you will need to seek professional advice from a suitably qualified patent attorney to determine the patentability of the web app. If you are successful in obtaining a patent, you will obtain exclusive rights to monopolise the product covered by the patent.
Please also seehttp://www.ipaustralia.gov.au/get-the-right-ip/patents/about-patents/what-can-be-patented/patents-for-computer-related-inventions/ for information pertaining to software patents.