Its probably worth mentioning that using miles for an economy flight will likely turn out to be far more expensive than just buying a sale fare to Australia. In general using your miles for an economy redemption is poor value. Unless you are going 1-way only (i.e. not returning), or wanting to take advantage of a stopover or two (e.g. stopping in Bangkok on the way out, and Singapore on the way back) you will probably find it cheaper to look on flightcentre.co.uk or statravel.co.uk who will give better prices.
For example, if I ignore availability (which may be hard to find), if you did the most direct route, of MAN (Manchester)-SIN (Singapore)-SYD (Sydney) it will cost;
25,000 miles and £170 cash plus £190 of taxes. Turned into a return flight it is 50k miles and a total of ~£690 tax/cash. At the moment on
flightcentre.co.uk you can get a return economy fare for £100 less from Manchester on Emirates and save your miles (Not a completely fair comparison as this sale expires at the end of June, but kinda proves my point).
However, if you were to go in business it becomes a lot better value. Doing the above Manchester-Sydney return via Singapore (Note that finding availability on this route will be difficult/impossible due to Singapore Airlines blocking availability for business class on some of these cabins but this is to demonstrate a point) would be;
75,000 miles and approximately £870 of taxes and cash
The cheapest flightcentre fare for business class is £2200 - so a much bigger saving!