'Keen as mustard' is an Australianism meaning (in essence) 'very enthusiastic'.

It's origns lie in the fact that once upon a time, it was pretty much impossible to get any commercially made mustard in Australia except for Keen's Mustard Powder. Thus, mustard was by its very definition 'Keen's' and by some strange aussie leap of logic, anything that was 'keen' was also mustard.

All very strange.

There has been some question as to which came first, the phrase or the mustard, but after checking with a few elderly aussies, I'm pretty sure the mustard came first.

This little turn of phrase is part of the greater aussie tendency to create turns of phrase based on the 'X as a Y' framework.

Keen's is serious mustard.

Source:http://www.aussieslang.com/slang/australian-slang-k.asp