1. Rent Increase
You must be given 60 days written notice of a rent increase. The notice must state from when the increase applies, and the increased rent. (note it can’t be retrospective.
If you don’t get written notice, or the notice isn’t given in writing, you don’t have to pay the increased rent and should continue to pay your current rent.
If the landlord wants to increase rent they have to serve you with a proper written notice. If they did not serve you with proper notice in relation to the last rent increase, then it doesn’t apply, and you should not have paid the increased rent. If you did pay the extra money, then they owe you that additional money you paid. I suggest you contact them with the calculations and advise them of any adjustments due to you on vacation of the property. If they muck you around, contact Fair Trading. (do not deduct it from any rent due)
https://www.tenants.org.au/factsheet-04-rent-increases
2. Access to the property
Re them trying to get in while you were there, I would have just opened the door and challenged them. Attempting to access your rental property with no notice is trespass (in the absence of an emergency).
https://www.tenants.org.au/factsheet-08-access-and-privacy
The agent can show the property to prospective tenants in the 14 days prior to you leaving, a reasonable number of times, provided they give you reasonable notice. So they can’t just turn up and expect to have access. What constitutes reasonable can vary, but in terms of notice I would say at least 24 hours. I would contact the agent (in writing) and say:
I note that you may intend to show the property to prospective tenants during the last 14 days of my tenancy. As you know, reasonable notice of entry must be given, therefore I expect you to provide me with at least 24 hours notice of any intended access to the property in order to show it to prospective tenants. I also note that any access by you without notice and/or outside of a valid purpose is not permitted. Thank you for your co-operation.