The ACT Supreme Court judge who heard the man's case has called on the assembly to quickly fix the problem.

"Sometimes when cleaning up you throw out something useful," Associate Justice David Mossop said in a decision on Friday.

"It is a matter deserving of prompt legislative attention."

The problem arose after adopted man John* reconnected with his biological father in Germany and sought to have the relationship formally recognised in the European country.

But German courts refused to recognise the relationship until the Australian adoption decision was reversed.

John, 44, was born in West Germany in 1972, shortly after his mother had her marriage to her husband annulled.

She took John with her to Australia that year, and later remarried.

His step-father formally adopted John in 1981, because the couple wanted him to have "equal status" with the son they had had together. The couple later divorced.

Then, when visiting relatives in Germany in 1999, John met a man who was later confirmed by DNA to be his biological father.

John changed his last name to match that of his biological father, and the court heard he had become much closer to him than he was to his adoptive father.

But technical amendments made in 2008 to laws governing adoptions had the unintended consequence of removing the court's power to undo an adoption, Justice Mossop said.

That consequence only affected adoptions made under earlier adoption laws, which were repealed and replaced in 1993.

"This was clearly a consequence not intended by the legislature.

"It is not a matter which can be remedied by the Court through any orthodox process of statutory interpretation. It is therefore a matter which needs to be addressed by the Legislative Assembly."

The judge has adjourned the matter until the law is fixed.

*Not the man's real name.