Attorney-General George Brandis has defended as within his rights spending nearly $13,000 of taxpayer funds over the past four years on his personal library.

"All of the acquisitions are clearly within the entitlement," Senator Brandis told Fairfax Media on Tuesday, two days after paying back nearly $1700 in entitlements that he claimed after attending the 2011 wedding of radio announcer Michael Smith.

Senator Brandis and Nationals MP Barnaby Joyce could face an investigation into their use of travel entitlements for the Smith wedding. On Monday, Mr Joyce, the Agriculture Minister, said he had sent a cheque for about $650 to the Department of Finance.

The funds cover the use of a Commonwealth car to drive him to and from the wedding, held on the Central Coast of NSW.