The PSNI and election officials have issued a warning to voters in Foyle after a post on Facebook suggested electoral fraud using stolen votes was under way.
A number of Facebook pages posted pictures of around 100 proxy and postal application forms, along with a completed sample form using a fictional address of '123 Sinn Fein Street'.
They were discovered in a folder at the back of a shop in William Street in Londonderry, it was claimed.
Some individuals also claimed they fraudulently voted several times for the Sinn Fein candidate in the 2017 Westminster poll, Elisha McCallion.
The posts have been brought to the attention of the PSNI and the Electoral Office.
Following the 2017 General Election, it was found there had been an unusually high number of proxy and postal votes in the Foyle constituency - 17.5 times higher than the UK average.
Ms McCallion was elected with a tiny 169-vote majority.
SDLP MLA Mark H Durkan has called for tighter controls on proxy vote applications.
Mr Durkan said: "I have requested to see exactly what is in this folder that it is being suggested was found in William Street - but it is a fact the number of proxy and postal votes in Foyle during the last election was 17 times the UK average.
"In some wards, like Brandywell, Creggan and Ballymagroarty, it was even higher - as much as 27 times higher - so I think the Electoral Office needs to be asking for evidence as to why someone needs a proxy or postal vote.
"I saw on some of the social media posts discussing this alleged find of proxy voting forms, comments from people suggesting they voted four times for Sinn Fein in the last Westminster election.
"At that time I gave the Electoral Office a list of 250 names that were in my opinion clearly suspect and I was met with the response from them of 'What evidence do you have?' when they were the ones with the application forms and the evidence.
"I also gave the police a few examples but they told me if I gave them a list of names of suspected fraudulent votes, they couldn't go on what they called a 'fishing expedition' because it could be seen as political policing. But by not doing it, it is political non-policing."
DUP election candidate Gary Middleton said public confidence in the electoral system is crucial, especially in Foyle after previous concerns over fraudulent voting.
He said: "The recent revelations and concerns around postal and proxy voting in the Foyle constituency are particularly concerning because it is not the first time such issues have arisen in this area. Between the General Elections in 2015 and 2017 in Foyle there was a near 300% rise in the number of proxy votes with nearly 1,000 extra votes cast in this way.
"There is a real need to instil confidence in the electoral process, particularly in the Foyle constituency. That is why these most recent revelations must be investigated thoroughly and quickly.
"The Electoral Office must demonstrate that proper checks are in place to prevent fraudulent applications being processed and the PSNI must establish quickly whether any criminal actions have taken place.
"The Foyle seat will be one of the most keenly contested during this election. It is vital that the election is not decided by electoral fraud."
Chief Electoral officer Virginia McVea said her office had been contacted by the PSNI about the social media posts. She said: "I am aware of this and I have been engaging with the PSNI in relation to it but no one has to date come forward and made a complaint. Investigations must be done by the PSNI because I have no powers to secure evidence or compel people to interview but I have liaised with them.
"We have now entirely centralised the absent vote process. It is now dealt with entirely in Belfast and I oversee it directly.
"We are as robust and thorough as we can possibly be, bearing in mind people have a legal entitlement to apply for an absent vote. But it is very important that if people have concerns they contact the police who have the powers to investigate."
Derry City and Strabane District Commander, Superintendent Gordon McCalmont, said: "While no report has been made to police, we are aware of social media posts which make allegations about paperwork reportedly discovered in premises in the city centre of Derry/Londonderry. Enquiries are ongoing, and I would encourage anyone who has information about this to get in touch with us."
A Sinn Fein spokesperson said: "Sinn Fein, like other parties, assist and encourage people to exercise their democratic franchise and would oppose any attempt to deny that right."