You can change your IP quite easily from your own armchair if your network is set-up right. But why bother, I am confident that if every-one takes the survey just once then the real truth will emerge.
My only worry are the company stooges that will illegitimately try to bump up the bad results.
Surveymonkey is a little more sophisticated than that. It is not foolproof but certainly a lot more difficult to beat than merely changing IP addresses from the same computer.
From the numbers Steve is getting, any corruption of the results by duplicate or multiple entries will probably be fairly insignificant.
But to be really significant and truly indicative of the Australian airline industry I suspect he needs
at least 5% to 10% of all airline emplyees and that may mean a
minimum around 3,000 to 6,000 responses, spread across all airlines, all industry vocations, not just pilots and engineers.
The final results may require professional analysis and publication to get the populace, including politicians and public servants, to sit up and listen?