I think (and I should know because we negative gear!) that you can claim back at tax time:
1. The difference between your mortgage interest that you pay and the rent that you collect from your tenants. For example, this year we didn't have much to negative gear because interest rates were so low.
2. Any maintenance or improvements that you make to your investment property.

As for the issue of renovations, I would say yes in theory it is better to do them afterwards for tax reasons. However you need to then think about impact on your tenants and various rental tenancy laws. If you are planning *major* renovations, then it might be more complicated due to tenants being in the property, needing a livable safe space etc. That then might drive up the cost of the renovations and wipe out any potential tax saving.

Just something to investigate further.