It looks like your gallery might have been given fixed dimensions before it became part of a svManager or Showkase management system.
Here are a few things to try:
Make sure that you are using Showkase's 'Gallery Manager' theme ('Site -> Change Theme -> Gallery Manager') which displays galleries at 100% x 100% (filling the entire browser window, or in your case, your iframe).
If you are already using this theme, then try changing the theme to something different ('Boma' or 'Kosel') and then change it back to 'Gallery Manager'.
Now that your 'g2' gallery is part of Showkase, you could try rebuilding the gallery (edit the Juicebox Gallery page and go to 'Advanced -> Rebuild this Juicebox gallery').
Although the 'g2' gallery is already part of Showkase, it looks to have been originally part of svManager and became part of Showkase during the upgrade.
You could try importing the gallery into Showkase using the 'Site -> Import' functionality (to create a new Showkase page from the gallery) and then load this new page into your 'iframe'.
If none of the above automated suggestions work, then try opening your http://coromandelflavour.co.nz/showkase/g2/index.html page directly in a plain text editor and manually change the gallery width and height both to 100%.
A further solution (though not as clean as simply setting your gallery's dimensions to 100% x 100%) would be to add the following code to your 'showkase/_themes/manager/css/custom.css' file:
body.type-juicebox {
margin: 0 !important;
}
... to ensure that the margin on your page (visible when viewing http://coromandelflavour.co.nz/showkase/g2/index.html) is not displayed within the iframe.
One of these suggestions should hopfully solve your problem.