Auditors have warned the Assembly about the threat to its reputation from MLA expenses rows.
And the issue of bonuses paid by politicians to staff members was among those giving cause for concern.
The private warning was sounded by the Northern Ireland Audit Office, the province's public spending watchdog, in its annual Assembly audit findings.
It was conveyed in internal correspondence earlier this year, in the wake of adverse publicity about Stormont allowances.
In common with their counterparts in Westminster, MLAs have been under intense scrutiny on their taxpayer-funded payments.
There have been controversies about both constituency office rental expenses and the number of Stormont politicians with family members on their payrolls.
Audit Office correspondence with the Assembly for the financial year 2007/08 has now been released to the Belfast Telegraph, under freedom of information.
It contained warnings about “ reputational risk" to the fledgling Stormont institution in connection with MLA office expenses.
The Office Costs Allowance (OCA) available to Members has climbed over the past 18 months from £48,000 to £72,000. It is primarily used for staffing, renting and equipping constituency advice centres.
In its findings for 2007/08, the Audit Office raised a number of potential areas of concern, including bonuses for MLA employees.
It also noted “increased public concern” over the number of Members with relatives on their staff teams.
“In 2007/08, £68,385 was paid by Members to their support staff in the form of a bonus payment,” the document said.
“Whilst the payment of a bonus to support staff is permitted in the Members Handbook, there is little guidance as to when it is appropriate to make a bonus payment.
“In addition, the Members Handbook requests that the Personnel Office is provided with reasons as to why it is appropriate to pay a bonus. In practice, we found that detailed information is not always provided to the Personnel Office.”
The Audit Office continued: “There is a risk that, in the absence of detailed procedures in this area, public money is perceived as being used inappropriately.
“This risk is especially high in light of increased public concern over using family members as support staff and also given that many support staff do not have contracts of employment.”
Among the other points cited by the document were a duplicate expenses payment of £176 and overpayments of allowances to a number of Members.
"We remain concerned that there is a reputational risk to the NI Assembly, especially as public interest in the use of OCA has increased since restoration, and we have made a number of recommendations in this regard,” the audit document added.
The Assembly's official response to the findings is included in the newly-released internal documentation.
An Audit Office recommendation that further guidance be issued on bonus payments was among those accepted by Stormont officials.
It was also stressed by the Assembly that only “relatively small” amounts - totalling £4,226 - had been overpaid under the OCA system and that recovery action was taken immediately.
A spokesperson for the Assembly said:
“The NIAO, in their report, commented on the high quality of the accounts submitted for the period 2007/08.
“The NIAO provided the Northern Ireland Assembly with a clean audit opinion and stated that 'the expenditure and income have been applied to the purposes intended by the Assembly and the financial transactions conform to the authorities which govern them’.”
MLAs can claim for tea, coffee and tv
Assembly members can claim for everything from satellite TV installation to tea and biscuits under the office expenses system.
The maximum sum available to MLAs annually from the Office Costs Allowance (OCA) was increased from £48,000 to £70,000 last year.
That decision was taken by Secretary of State Peter Hain following the deal to restore devolution at Stormont.
The OCA climbed by a further £2,660 earlier this year, in line with the Retail Price Index measurement for inflation. The traditionally lower Consumer Prices Index measurement is normally favoured in government circles – for example when setting public sector pay.
OCA is used by MLAs to pay staff, rent constituency office premises, and cover running costs like heating, lighting, phone and rates bills.
Further areas of allowable expenditure are detailed in the official Members Handbook, a document obtained by this newspaper.
Permissible OCA expenses include office furniture and IT equipment, “limited refurbishment” costs, and stationery “strictly in relation to a Member’s Assembly business”.
Members can also claim “reasonable expenditure on tea, coffee and biscuits for staff and visitors” at their constituency offices, the document states.
On TV and radio equipment, the Handbook says: “The cost of a radio, a television and the installation of satellite TV may be met from OCA.
“Ongoing subscription charges for satellite TV may also be met... provided the package installed is of a news and current affairs related nature.”
The cost of sports and movie channels can’t be claimed, it makes clear.
The Belfast Telegraph launched an Open Stormont campaign earlier this year to promote greater accountability and transparency at the Assembly.