DUBAI, Oct 27 (Reuters) - Saudi Arabia's central bank said on Thursday it was introducing a new money market instrument, a 90-day repurchase agreement, to improve the stability of the domestic monetary system.

The central bank also said it was lowering the maximum volume for its Treasury bill issues to 3 billion riyals ($800 million) per week from 9 billion riyals.

The steps were the latest in a series of efforts by Saudi authorities over the past couple of months to ease domestic liquidity conditions, which have been strained by government borrowing to finance a big budget deficit caused by low oil prices.

(Reporting by Andrew Torchia; Editing by Kevin Liffey) ((andrew.torchia@thomsonreuters.com; +9715 6681 7277; Reuters Messaging: andrew.torchia.thomsonreuters.com@reuters.net))