The Standards in Public Office Commission has confirmed it is to write to the Socialist Party seeking further information about its policy of using part of the salary of public representatives to fund the party work.
A spokesperson said the letter would point out the limits on donations by any individual, even if they are a TD or MEP, to a political party, and seek information on how the arrangement operates.
The Commission has already written to Sinn Féin in similar terms.
Elected representatives of both parties take only the average industrial wage from their salaries, and have said they use the rest to support party work, though it is not clear exactly how the arrangement is structured.
The Commission says it has no role in the separate issue of the use of Dáil travel expenses.
Oireachtas officials are considering whether the use of Dáil travel expenses to travel around the country to campaign against the Household Charge is within the rules.
A spokesperson for the Houses of the Oireachtas said consideration was continuing, and it was not clear when a decision will be made.
The issue was raised when Socialist Party TD Clare Daly said she was using some of her travel expenses for this purpose.
This morning, her colleague Joe Higgins told RTÉ's Today with Pat Kenny that it was "absolutely justifiable" to use the travel allowance to build a people's campaign of opposition to what he called a "vicious austerity tax."
He said the campaign against the charges was not a Socialist Party campaign, but a people's movement.
He described it as the biggest campaign of opposition and of civil disobedience that the State has seen.
Deputy Higgins said it was time the political establishment and sectors of the media began to realise the depth of feeling among people about the charges.
Meanwhile, Chair of the Household Charge Project Board Jackie Maguire said there has been a "steady flow" of payments online since the initial deadline for payments passed at the end of March.
Reminder letters are due to start arriving in the post today for those who have not yet paid the charge.
On RTÉ's Morning Ireland, Ms Maguire said the letters remind people that the charge now due is €114.
The board will pursue those who have not paid and if people had to be taken to court to force them to pay then such action would be taken, she added.