I have a 73 Sportster and I can't find any history on it because it doesn't have a VIN. I'm thinking I might have trouble with rego in NSW without import documents. Has anyone here had experience with this?

I thought that in some cases of old bikes the Engine # was essentially the VIN.  But thats grey in my head...

I think you may be right. I tried searching using that but all of the search sites need a 17 number VIN 

It can be tricky, states differ a bit so I'd contact your state rego authority and see what the process is, if it's a grey import you will need a Vehicle Import Approval document to begin with I think.

Do you actually know it's history, how long it's been in country?

I'll check with service NSW. The only thing I know Is it was bought from an auction and it was in QLD