Australian Maddinson used a bat that failed a bat-gauge test during Durham's game against Derbyshire earlier this month. The equipment was inspected by the umpire before the left-hander was asked to continue with another bat.
"Durham have accepted a 10-point deduction from the Cricket Discipline Commission (CDC) after batter Nic Maddinson admitted a breach of ECB Directives 3.2 and 3.3," England's Cricket Discipline Commission (CDC) said in a statement.
"The CDC considered the points put in mitigation by Durham CCC and Maddinson, that the offence was committed unintentionally, but confirmed that the matter had to be dealt on a strict liability basis."
Marylebone Cricket Club rules dictate that a bat must not be longer than 96.52cm from the handle down and its dimensions should not exceed 10.8cm for width, 6.7cm for depth and 4cm for its edges.