Federal Labor MP Bill Shorten has called for an investigation into claims two Coalition frontbenchers used thousands of dollars in taxpayer entitlements to attend a wedding in Sydney.

Attorney-General George Brandis and Agriculture Minister Barnaby Joyce reportedly claimed over $3,000 in travel expenses to attend the wedding of radio presenter Michael Smith in 2011.

Senator Brandis has told the Fairfax media that he used the wedding as an opportunity to collaborate with Mr Smith over his work covering the Health Services Union scandal involving former MP Craig Thomson.

Mr Shorten says the reason given does not justify the use of taxpayers' money.

"I think George Brandis and Barnaby Joyce will most likely have to refund the money," Mr Shorten said.

"I think there should be an investigation. It is not normal to say that the reason why you get the taxpayer to support you to go to a wedding is so that you can network with journalists."

Fairfax newspapers reported that the cost of flights, hire cars and incidental expenses were among travel expenses lodged with the Department of Finance.

Mr Joyce rejected the Fairfax report, saying he may have used a Commonwealth car on the day, but that he did not claim flights or accommodation.

"The only thing I can see in this, and it was two years ago, was the use of a COMCAR on the same day as the wedding," Mr Joyce said.

"I will now dig back and do what I can to find out about that and if there's some ambiguity I'll pay it back, but the idea I claimed thousands of dollars is just wrong."

Mr Shorten told ABC's Insiders program he does not want to "ultimately prejudge the full consequences".

"There may be an explanation. I don't know all the facts, but should there be an explanation? Yes."

Meanwhile, the radio announcer at the centre of the controversy has defended the actions of the politicians on his website.

Mr Smith says the cost of the then-Senators' transport to the wedding was paid for by he and his wife.

He says any expense claims were justified because both Senators engaged with journalists at the wedding and their attendance at the celebration "did not demur in any way from their paid elected role as prominent federal parliamentarians".

Mr Smith asked of Fairfax: "Is it your interpretation of the [guidelines] that where an expense is incurred in speaking with local media that cost should be personally borne by the member?"

According to the federal Department of Finance and Deregulation's entitlements handbook, expenses for official business such as "meetings of a government advisory committee or taskforce" or "functions representing a minister of presiding officer" are allowed for. Meetings with journalists and other members of the media are not sanctioned under the handbook's guidelines.