I too live and invest in Darwin.  As a previous poster has pointed ouit Darwin is powering ahead and not showing signs of slowing.  Population is growing , shortage of housing  and many large infrastructure projects in the offing and underway,  Could you ask for more?  If you are planning on staying here for a few years it is a no brainer.  Cost of servicing a mortgage or paying rent is fairly similar so the only answer is buy.  Forget about financial advisers for help in property investing.  Never met a FA yet who wants to know about property  – no commissions in it for them.  Beware of property spruikers  seminars too (not that we get many of them up here!)  Best is to educate yourself by reading and mixing with other property investors and get to know a few good RE agents and  buyers agents (Darwin only has one buyers agent at present)  They should be able to give you some guidance.  In my opinion, you would be hard pressed to find a better place to invest in resi property in Aus than Darwin at the moment .   

Try mopping the tiles with hydrochloric acid.  It is what builders use to clean brickwork and is also used to clean a concrete floor prior to painting.  Mop on and it should fizz a bit.  Leave on for 20 mins or so and maybe do again if still dirty.  Mop off all residue with clean water a few times,  Try not to splash acid!  This will clean grout and also etch it .  You can then squeegee over a thin layer of grout, then sponge over.   You are not really replacing the old grout just cleaning and freshening up.  Use the same coloured grout too.Usually works well………………..