Western Sydney parking rangers who marked tyres with crayon instead of chalk have made a potentially costly mistake as a local council scrambles to close a loophole that could trigger almost $2 million in ticket refunds.

Key points: Parking inspectors in Parramatta use crayon to mark the tyres of vehicles before they issue fines

Parking inspectors in Parramatta use crayon to mark the tyres of vehicles before they issue fines However, the Council's Parking Enforcement Policy refers only to "chalk"

However, the Council's Parking Enforcement Policy refers only to "chalk" The loophole could mean infringements issued after September last year are invalid

The oversight has raised questions about whether parking infringement notices issued after September last year were unlawful, potentially giving drivers the last laugh.

The City of Parramatta Council is concerned enough to have sought legal advice and consider changing its parking enforcement rules.

A motion about the problem in June was kept confidential, but details appeared in the business papers published ahead of the next general council meeting, which will be held on Monday.

The papers said: "The operation of parking enforcement was discussed extensively and the reasons why crayon is the preferred substance for use when marking tyres for enforcing timed-related parking offences."

The Council's Parking Enforcement Policy refers only to "chalk" — not crayon — as the method for marking tyres.

The papers said fines issued from September 2017 — when the most recent NSW local government elections were held — could be affected.

NSW's Road Transport Act, however, refers to using "crayon, chalk or similar substance" to write the time on the tyres and determine whether the vehicle has been parked too long in a time-restricted space.

Parking inspectors use crayon to determine how long a car has been parked. ( ABC News: Malcolm Sutton )

Based on legal advice, Council officers reported that "using crayon as a marker rather than chalk, does not lead to the conclusion that the Council officers have acted unlawfully (beyond power) or that the consequent infringement notices are invalid".

If the fines issued using industrial crayon were found to be unlawful, ratepayers would have to stump up about $1.9 million in refunds.

"This issue came about when a councillor put forward a Notice of Motion at a Council Meeting on 25 June relating to the use of crayon instead of chalk by Council’s parking rangers," a City of Parramatta Council spokesperson said.

"Operationally City of Parramatta Council uses crayon as do most other parking enforcement agencies across NSW, including the NSW Police."

The Council would also need to somehow identify and locate the vehicle owners, despite not having the authority to access the Roads and Maritime Services database.

Despite the legal advice, the Council is moving to close off any potential loophole.

It has sought to replace the terms "to chalk" and "chalking, with "to mark" and "marking", among other changes to clarify the parking fines policy.