Increased deposits at a bank does not increase their capability to make loans. The only requirement is the maximum equity to loan ratio.
I wouldn't dismiss the capital requirement issue quite so lightly.

The current trend is to tighten these ratios which could well constrain the ability of some banks to increase their lending. Kiwibank would be in this category, IMO, as NZ Post (the govt) wouldn't welcome - or be able to afford? - the call for increased capital to support increased lending, regardless of how easy or difficult it may be to raise the deposits to fund them. I can see the new Heartland bank having the same problem down the track.

All a bit off the subject of a capital gains tax. For what it's worth, I'd like to see a comprehensive tax along the lines of the Aussie one as soon as possible to take some pressure off a system which relies overly much on taxing income from earned income/company profits.