Medibank Private has excellent travel insurance at good rates and members get a further 10% discount. No travel insurance is very expensive for 20 days, but don't skimp on it and get cheap instead of good.

EDIT:
I just did some comparisons and they came out much cheaper than the others.

I used Ace I think they could be british but also in Australia Sydney. I took a good look at others as the prices varied quite a bit . they where cheaper than any others i could find and they read well. I booked it on line. travel insurance for America or Canada is an absolute must. If you landed in Hospital you would be paying it back for years to come if you had no insurance