Are you taking tax advice on your Australian position? Is work paying relocation? Living-away-from-home-allowance? Is your move temporary or long term? The tax implications are quite different. Generally, Australian tax resident's are taxed on their worldwide income. However, persons on a 457 visa may be considered "temporary residents". Temporary residents are usually only taxed on their Australian sourced income. So you may not have extra tax in Australian on your UK rental income. If obtain Australian permanent residence or citizenship you will no longer be "temporary resident".
In relation to your UK position, are you going to live in it for a bit? Sounds like you have ran out of time. If you live in it you get the main resident exemption. This can apply for 18-36 months after you move out.
You may be a UK tax resident for the whole of the year 6 April to 5 April 2015.
Correct you need to deal with the NRLS. Register for it. Fill in the election which you submit to HMRC requesting that you be taxed via tax returns rather than a withholding tax. Are you using a letting agent? If so they should be able to give some guidance.
Generally, the tax levied rents derived by non-residents is 20%. So you may be able to get a good tax outcome between the two countries.
Elise Ashmead