Most of the major banks will offer these kind of deals on PPOR mortgages, typically you will pay 3 or 4 hundred dollars a year to get this. They tend to be called 'Professional packages'
ANZ call theirs Breakfree, you get 0.7% off std variable and 100% offset & up to five loans (IP & LOC etc) for no extra costs, so if you have a mortgage any cash in the offset is effectively earning you money at lenders interest rate and its not taxed at the moment at typical 40% tax rate, its worth about 7 or 8% pa
NAB do a similar deal, i think this one is called 'Choice'
I think to pay for flexability is worth it, like I pay a premium of 0.16% over the homeloan rate for a line of credit so you can whack cash in and out and your only paying for what your using. And when you concider after tax it's only .0096% a tad under a K per mill. And it pays for itself because if you see a bargain and you're a cash buyer and can act pretty quickly. The last thing you want is too have to rush arround organising money if you see something shares or real estate that you want to snap up fast. Also when you have spare cash it's parked at an effective rate as you say.
Cheers
Gary