Can putting an application form for a new job in with the business mail be considered an example of theft / fraud at work?
I guess potentially it could, you mean if your employer paid the postage as it went in their mail stream, as It's not company mail? While technically it could, I would think that unless you did hundreds of them there would be no chance of prosecution, though of course they could use it to fire you if they found it as it is definitely not what the business mail is meant for.
In NZ it would be "theft as a servant" although a lot of companies turn a blind eye. However since you are applying for a job and therefore planning to leave your job it's morally corrupt to expect them to pay for it. If you can't afford postage yet you are employed then where does your money go? I wouldn't employ someone who couldn't manage their money - how can I expect them to manage their time, be productive and an asset to my company.
I cannot understand why anyone should want to do something like that. It may seem to be a small matter but actually, it is a reflection of the heart's content that doesn't smell too good. I personally will view such an act as a warning sign that the person in question should not be trusted to be in my employ. In my assessment of people, I pay close attention to the (seemingly) small things. To answer your question: Yes, such an act is an attempt to steal and defraud your employer.
It is quite simple. It is THEFT. And if you would steal from your current employer, HOW could you expect your new employer to ever trust you?