How do I repeat a calendar event every x seconds in macOS Sierra?
I know this won't be the answer you were looking for.

In your question you stated: 


  I then exported an event as an ICS file, and then edited the ICS file in TextEdit, but I must've got the code wrong, as it didn't re-import
  into macOS Calendar with the correct repeat interval.


Unfortunately you probably didn't get the code wrong. 

While you are correct that the ICS specification does indeed support intervals down to the second, the problem is that the default macOS Calendar app doesn't support this (at least not in terms of repeat or recurrence values). So, what you want to do is not possible with the default macOS Calendar app. In fact, the problem you have is twofold, as the Calendar app does not recognise intervals:


shorter than a day
not represented as whole numbers


To explain this further, the first problem above would not necessarily be an issue if the Calendar app accepted a decimal value to enough decimal points, because then you could enter an event that repeated every 29.53 days. 

However, even if the app did accept a decimal value, it'd likely be limited to no more than one decimal place, which won't be precise enough when dealing with days. 

Unfortunately, MS Outlook won't work for you either (it doesn't accept anything less than daily intervals and even though you can enter decimal values, it just rounds them up or down).

And Fantastical, while an excellent calendar app for macOS, is as limited as the default calendar in this regard.

Instead, what you will need to do is:


find a solution that registers intervals shorter than a day, or works with decimal values, or
download one of the existing astronomical calendars for this purpose and then manually edit the ICS file to change the time zone to suit the various Australian ones you need.


Sorry this isn't what you were looking for, but maybe the second 'download and edit' option will work for you without too much effort.