The listed coordinates are not for the cache, but for the outdoor public viewing area at Perth International Airport
The cache is a black 200mL Sistema which can be located based on information you'll find on the viewing platform. Due to the lack of appropriate nearby hiding spots, the final cache is close as the crow flies but in reality, it's a bit of a drive. It contains a logbook, pencil and sharpener but has a little bit of extra room available if you'd like to bring something along.
Opening on the 16th December 2011, the viewing area is located adjacent to the threshold of runway 03. It provides great views of aircraft taxiing, taking off and landing, as well as most of the main runway, the international terminal and the air traffic control tower. The best time to visit is when there's a northerly or nor-easterly wind blowing, but you'll generally be able to see something regardless.
The area has a few facts about the history of Perth airport and some allusions to aviation history in general. You can read all about it on the information signs. While your at it, gather the following information to guide you to the cache:
A = The second digit on the second line of the sign welcoming you to the viewing area
B = The second digit on the very last line of the sign pertaining to the Wright Brothers first flight
C = The second digit on the first line of the sign pertaining to the history of the airport
D = The last digit on the second line of the sign about the airport's redevelopment
E = The most common digit on the sign about the aircraft that marked Australia's entry into the jet age
F = The third digit on the second line of the sign pertaining to the environment
G = The second digit of the only number on the sign about the Aboriginal landowners
The cache can be found at S 31 57.[F/A][E-A][C-B] E 115 57.[D-G][A+B][F-D]
For extra credit, upload a photo of yourself or your GPS with one of the following:
3 Points* - Airborne aircraft
2 Points* - Aircraft on the ground
1 Point* - Air traffic control tower
The viewing area is open daily between 6.30am and 7.00pm (October to March) and between 7.30am and 5.30pm (April to September). Ear plugs are reportedly available at the site but I haven't seen them. Please take due care to protect your hearing, jets can be loud!
* Points are arbitrary, hold no value, and cannot be redeemed for anything, ever, but they're fun to collect 
Congrats to R0gue on FTF, that's a 2/2 streak on my caches now!