JB
Make a list of key characteristics of your paddle stroke and your paddling requirements and talk to a reputable paddle shop. The best paddle shops will carry multiple brands and models and will let you test paddle different types. It might take a little searching, but finding a full service paddle shop like this will pay dividends in your level of satisfaction with your purchase. If you plan on 20K trips, you will want to make sure that you are very comfortable with the paddle before you buy. Either that or make sure they have a very good return policy.
When listing stroke features and requirements take into account:
1)Level of fitness

verall strength and joint health (wrists, elbow, shoulder). Wider/larger blades require more strength and put more stress on joints.
2)Height: I know there is a general rule for paddle length vs height. It doesn't always hold true for fishing kayaks. The broader beam of most fishing kayaks calls for a longer shaft.
3)Paddling angle: At what angle are you most comfortable holding your paddle. Shallow angle=greater endurance/higher angle=greater power. This will determine if you want a narrow touring blade or a wider blade that provides more bite.
4) Straight vs bent shaft. Some people find that the bent shaft is more comfortable and easier on the wrists. I don't think that there is any conclusive evidence for or against; it is strictly a personal preference. You must paddle with a bent shaft for a while to decide if it is right for you.
5) Straight or feathered shaft: Is the wind a significant factor when you paddle? Are you already using a feathered shaft. Do you paddle with a high or low angle paddle stroke?
6) One or two piece shaft: Do you want/need an adjustable feather angle? Do you have room to transport a single piece paddle. Obviously, a one-piece shaft will be stronger. Carefully check the fit of a two-piece shaft. A loose fit will only get worse.
7) Where will you be paddling: If you have to launch and land in the surf, you might want a more aggressive blade that will give you the power for dashes between sets. You'll want a nice strong shaft/blade if you will be bracing with the paddle.
8 ) Paddle weight: A lighter paddle is a joy on those longer trips. Carbon fiber shafts can't be beat for weight and strength.
9) Shaft/blade material: Will you be paddling amongst rocks and piers, or sand and mud. Get a paddle that can handle the conditions that you normally experience.
I'm sure that there are some things that I am forgetting. Take your time and try as many paddles as you can. On a long trip, a bad paddle will remind you how much it sucks with every stroke.
When I bought my first kayak I got a paddle thrown in as part of the deal. It sucked. I was new to kayaking and had no idea what I should look for in a paddle. I quickly learned a valuable lesson; the paddle is just as important as the yak. Since then I've gone through three different paddles. As I've gained experience paddling, I've refined my requirements for my paddle. I am extremely fortunate in that I have a full service kayak shop close to where I work. I've been able to try out new kayaks and paddles before making the decision to buy. The owner lent me his own paddle for a week when I told him that I was interested in trying out a bent shaft. I had a chance to paddle it on a flat-calm bay and also take it out in some snotty conditions offshore before making a decision. You can't beat that. If you can't find a good paddle shop that will let you try out the products, see if you can borrow one from a friend. Paddles, kayaks and PFDs really do need to be tested for comfort before buying.
Here are some links to paddle manufacturers that you might find helpful.
http://wernerpaddles.com/touring.html I'm a fan of Werner Paddles. I've got two carbon fiber Werner paddles that are just awesome.
I just got a new Ikelos
http://wernerpaddles.com/paddles/ikelos.html
Paddlesports Australia is a Werner dealer
http://www.paddlesports.com.au/
NZ dealer
http://www.nzkayakschool.com/sales.html
Here are two smaller paddle makers in the US. I don't know about availability in Australia, but they have interesting websites that provide some useful info
http://www.atpaddle.com/products/touring/TouringGateway.html
I think that Holy Mackerel uses one of these. You might PM him for his opinion. They have dealers in Australia and NZ
http://www.horizonline.com.au/index.htm
http://www.fergskayaks.co.nz/
Others
http://www.bendingbranches.com/choose_blade.html
http://www.aquabound.com/