There's a number of issues with your workings. Regarding...

There's a number of issues with your workings.

Regarding earnings, you're apparently using trailing rather than forward metrics. The market tends to be forward-looking and given GTK has guided to 15%+ earnings growth (and their guidance has historically proven to be conservative), a fairer measure would be to look at what FY18 earnings might be. Secondly, you should be reversing out the amortisation of intangibles in the NPAT number, as this more closely correlates to the free cash flow of the business.

If you perform both those steps, you start with NZ$14.3m FY17 NPATA, and assume 15% growth (bottom end of guidance), you get NZ$16.5m or A$15.3m vs. market cap of $465m, so your scary "PE of 40" drops to a still high but not quite so scary 30x.

I think you'll be waiting quite a while for this to trade at 15x earnings - companies that are organically growing earnings in the high single digits or low double digits generally don't trade at 15x earnings. GTK is kind of a case in point - it was somewhere around the 15-18x earnings mark for the first year or so of its listed life, then once they had delivered 1.5-2 years of high single digit / low double digit organic growth (plus intelligently acquired growth), and guided a continuation of this growth profile, the stock's multiple lifted significantly to its current ~25-30x.