Fresh doubts have been cast over Labor's claims about the strength of its citizenship verification procedures after legal advice from a leading citizenship expert found an MP was a dual citizen when she was first elected in 2015.

According to former British home office lawyer Phillip Gamble, Labor senator Katy Gallagher, who earlier this year acknowledged she was eligible for British citizenship, would have been a British citizen when she first took her seat in Parliament.

The Labor Party has insisted its protocols are beyond reproach as Opposition Leader Bill Shorten and Prime Minister Malcolm Turnbull prepare to discuss the details of the proposed citizenship register in Melbourne on Tuesday.

Senator Gallagher renounced her right to British citizenship prior to the 2016 election in what she described as "an abundance of caution" but Mr Gamble, a UK citizenship expert, says she was already a British citizen when selected to fill a casual vacancy in 2015 as a result of acquiring it automatically from her British-born father.