Cabinet minister Bridget McKenzie will tomorrow update the Senate gift register after admitting she failed to disclose free travel to campaign for Deputy Prime Minister Barnaby Joyce.

The then Nationals backbencher travelled to Tamworth in December for the final days of Mr Joyce's by-election in New England, having lost his seat in the Parliament because of the citizenship saga.

Sorry, this video has expired Barnaby Joyce declares victory in New England

Senator McKenzie became her party's deputy leader and the minister responsible for the sport, rural health and regional communication portfolios in the days after Mr Joyce's victory.

On Friday, the ABC contacted Senator McKenzie's office seeking clarity on how she travelled between Canberra and Tamworth.

It came after the ABC confirmed the airline Regional Express (Rex) offered charter services for National Party politicians from Canberra to Tamworth on Friday, December 1, with a return flight to Canberra on Sunday, December 3.

A spokeswoman admitted Senator McKenzie had received free travel and said the failure to disclose it had been an oversight within the senator's office.

Melbourne Spring Racing Carnival

The spokeswoman said it appeared, based on diary entries, that Senator McKenzie had travelled on one free flight and the registry would be updated "first thing Monday".

Senators are required to update their Statement of Registerable Interests within 35 days.

Senator McKenzie last updated the register on November 13, 2017, when she declared free tickets to the AFL Grand Final, hospitality during the Melbourne Spring Racing Carnival and accommodation for dairy awards in Adelaide.

Queensland's Michelle Landy was the first Nationals MP to disclose the free travel, which she said was worth $500.

That occurred on December 5. NSW senator John Williams updated his register the next day, declaring he travelled on the free Rex flights.

Four other National Party MPs made declarations later in December — Queensland's David Littleproud, who has since become a Cabinet minister, and Victoria's Darren Chester, Damian Drum and Andrew Broad.

Airline making political donations

A Rex spokeswoman said the airline made donations to political parties, which the parties determined how they would use.

"The donations are normally provided as a travel budget for Rex flight," she said.

"It is up to each party to decide how they wish to make use of the budget."

The spokeswoman said Rex had previously donated to the Liberal, National and Labor parties.

Disclosures published last week showed Rex donated $17,379 to the National Party in the 2016-17 financial year, which included the last federal election.

There were no donations declared to the Liberal or Labor parties during that period.

Nationals paying their own way

The ABC has also confirmed that Resources Minister Matt Canavan and MPs Llew O'Brien, Luke Hartsuyker, David Gillespie, Michael McCormack, Andrew Gee and Kevin Hogan travelled to Tamworth at their own expense.

Of the party's remaining federal politicians — Keith Pitt, George Christensen, Nigel Scullion, Barry O'Sullivan, Mark Coulton and Ken O'Dowd — the ABC has confirmed they did not attend the by-election celebrations in Tamworth.