I think signing some kind of NDA and non-compete would make sense but still, is that all there is to protect yourself from employee just sharing your source code or inner-workings of the system and whatnot with the world or competitors? I doubt that your employee from New Zealand will care much about your product/company in UK, for example, anyway. So the legality has little enforcing strength to begin with.
Unless you're pulling an unscrupulous rando of fiver, it's better to presume that your employee is a professional and won't do that, rather than burn a large amount of resources protecting your source code. What could I actually do as a programmer with, say, a copy of the Photoshop source code?
if your SaaS app is, say, some intricate detail in the UK VAT system, how much could the code even be worth to someone in Australia? even if they did steal your code, do you have the energy to make an Australian subsidiary and pursue that? the opportunity cost would be high.
for that matter, if you're going to go down the route of not trusting your employees just to make sure no one wants to work for you even more, are you gonna drug test them as well? pay them what they're worth and treat them well, and they won't try and steal from you.
a standard employment contract should cover "don't steal our source code" without going to the lengths of being an NDA. that's not to say NDAs aren't used in the industry though. For what it’s worth, noncompete aren’t enforceable in California and that hasn’t seemed to stop its tech industry one bit.
Otherwise, don’t worry about it.
Being restrictive and paranoid towards your future hires is only going to guarantee that desperate employees work for you. Well-tenured industry veterans aren't even going to sit through the entire interview if you make them sign a non-compete.
Ultimately your brand will be the most valuable thing, but if you don't have an established one I think it's fair enough to take precautions with someone you've never met and in a different jurisdiction.
I would expect a 'professional' to understand.