Doubt has been cast on Bill Shorten’s selection as Labor leader amid reports that the mailing addresses for dozens of ballot papers were changed before the leadership vote.

According to Fairfax Media, an ALP tribunal decision found 50 members’ addresses were altered following a request from a staff member of Senator Sam Dastyari.

The media group said at least 20 were changed to the address of Auburn councillor Hicham Zraika, who on Monday was suspended after being found to have engaged in “unworthy conduct”, including branch stacking.

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The tribunal found Mr Zraika should have alerted the party when he received the ballot papers destined for other party members “because it was necessary to preserve the integrity of the [federal Labor] leadership ballot”.

Party members are understood to be concerned that the case is not a one-off.

Mr Shorten was elected leader in October, 2013, after beating Anthony Albanese in a 55-31 caucus vote.

He earlier lost a wider membership vote, which backed Mr Albanese 60-40 per cent.