But in May last year, he received another letter from the ACT Revenue Office asking for confirmation of the rental income he had declared in his tax returns from 2005 to 2016.

Mr Rovere complied and forwarded copies of his previous correspondence and advice to the Revenue Office as well.

In September, he received a notice for outstanding land tax totalling $10,641 for the period 2005 to 2018.

Mr Rovere tried to argue that his failure to pay was the result of incorrect government advice, and that the debt should be struck off.

However the ACT Revenue Commissioner rejected his claim and Mr Rovere was forced to pay the bill.

Mr Rovere believes that was unfair, as he had been relying on "bad advice" from the government.

While the attached flat is currently rented at a rate of $180 per week, he is retired and the bill put a dent in his finances.

"$11,000 is a lot of money for me. I live frugally, I don't go on holidays," he said.

"I'm prepared to pay from now on whatever charge because I accept things change, I believe in taxation. Nobody is happy to pay taxes, I believe every citizen has a duty to pay.

"My thing is not that I refuse to pay, but that they give me bad advice. They changed the rules. If electricity is going up or things change, they notify you. I already paid but I find it unjust.

"I don't want to stir up trouble but I think I've been treated unfairly."

Land tax is payable on any property that is not the owner's principal place of residence, except in some circumstances.

Revenue officials detect missing land tax by comparing records with other government agencies, looking for landlords who tell the Australian Taxation Office about their rental income but fail to pay tax to the ACT government on their investment properties.

Landlords repaid $13 million to the territory last year after a push by the ACT Revenue Office to recover missing land tax.

The ACT government refused to comment on Mr Rovere's case on privacy grounds, but said under tax law, the onus was on taxpayers to be aware of their obligations.

"Since its commencement, the Land Tax Act 2004 has allowed for the determination of land tax on multiple dwellings even where they are adjoined," a spokeswoman said.

It throws up the question as to whether more granny flats across Canberra are also liable for land tax, especially with the new vacancy tax the territory introduced last year.

However it is believed that granny flats are only liable for the tax if they are rented.