I've found this http://www.insapipe.com/hotstream.htm which looks ok as the tapware/rose is a lot more robust than the standard electric showers. It's an 8kw unit, not sure what kind of pressure I'll be able to run the hot water at though.
Also looked at this http://www.trademe.co.nz/building-renovation/plumbing-gas/water-cylinders-heaters/auction-778134761.htm which looks like it attaches to the water supply and I can use the existing shower hardware. It's an 8.5kw unit and caps pressure at 3.5l per minute, is that enough for a reasonable shower?
I'm wondering if anybody may be able to let me know if they've used anything similar in the past and are there any better options than what I'm looking at? I'm keen to keep installation costs as low as possible.