The party, which has no seats in parliament, has 60 days to remedy the breach.
Earlier this month SODELPA, the largest opposition party in Fiji's parliament, announced Tabuya would stand as one of its candidates in the next election which is due next year.
She said it was the most pragmatic way to go given the five percent threshold for parties to gain a seat and she said the PDP had agreed not to put forward any candidates.
The two parties have been given seven days to provide Saneem with copies of the Memorandum of Understanding they signed earlier this month.
Saneem said certain requirements needed to be met if parties wanted to merge.
He said the parties may have exceeded deadlines for some of the procedural requirements for party mergers.
The elections supervisor said he had also referred a complaint to Fiji's anti-corruption body FICAC about Vijay Singh, another PDP office-holder, who had been appointed a vice president of SODELPA.
Saneem said the complaint was in relation to possible contravention of the rules which disallow dual party membership.
Photo file caption: Lynda Tabuya speaks to the press