Attorney-General George Brandis has paid back almost $1700 claimed in parliamentary expenses to go to the wedding of a radio shock jock.

Senator Brandis said although he believed the function was work related, there was "uncertainty" about the entitlement.

Senator Brandis and coalition colleague Barnaby Joyce have come under fire for claiming $3000 between them to attend the 2011 wedding of friend and former 2UE broadcaster Michael Smith, which was held at a hotel owned by John Singleton on the NSW Central Coast.

Senator Brandis had told Fairfax his attendance was "primarily professional", because Mr Smith had been covering scandals involving then Prime Minister Julia Gillard and Craig Thomson.

But in a subsequent letter to the Department of Finance he enclosed a cheque for $1683.06, saying while he considered that they were within parliamentary entitlements, he wanted to "resolve any uncertainty in favour of the taxpayer".

"I considered that those costs were within parliamentary entitlements, since they were incurred in the course of attendance at a function primarily for work-related purposes. I remain of that view," he wrote in the letter on Sunday.

"It is clear that the relevant criterion is the purpose of the travel, not the nature of the event.

"However, I accept there can be uncertainty about the circumstances in which attendance at a private function for work-related purposes is within the entitlement."

Comment is being sought from Mr Joyce, the new Agriculture Minister.

Labor leadership candidate Bill Shorten has called for an investigation into the matter, and noted the irony of the allegations against Senator Brandis.

He said the attorney-general had been one the coalition's "lead attack dogs" against former Speaker Peter Slipper, who is facing fraud charges over claims he abused his parliamentary expenses.

"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," Mr Shorten told ABC Television.

"I just wonder what George Brandis would say if it had been a Labor person.

"I just wonder if he'd call for a police investigation. Would he expect the same standard to be applied to him, if he had called for a parliamentary investigation?

"It is most unusual that you just go to a shock jock's wedding, who's got political views close to your own, and then we read reports in the media that all of us a paying for it."

Finance Minister Mathias Cormann defended his coalition colleagues, saying both had believed the claims were within their parliamentary entitlements.

"Claims that are made by politicians in terms of any type of public allowance need to be made with entitlement," Mr Cormann told Network Ten.

"At the time, the judgment was that it was. If there is change to this I'm sure that this will be worked through in the appropriate way, in the appropriate process, and if there are conflicts along the way, I'm sure they will be dealt with in the appropriate way as well."