ASPIRING treasurer Joe Hockey has been caught breaching parliamentary rules after failing to declare a family interest for almost his entire 17-year career in federal parliament.

Mr Hockey declared the directorship of Steel Harbour Pty Ltd held by his wife, Melissa Babbage, in May last year among a series of "new positions" under spouse declaration rules.

But business records show Ms Babbage was appointed to the role in 1998.

Pecuniary interest register declarations are supposed to be made within a month.

The revelations come after News Corp Australia revealed earlier this year Arthur Sinodinos, tipped to be Mr Hockey's right-hand man as finance minister in an Abbott government, failed to declare six company directorships in his first year in the upper house.

The directorship blunder has emerged ahead of a key economic debate showdown between Mr Hockey and Treasurer Chris Bowen at the National Press Club in Canberra on Wednesday.

Mr Hockey yesterday suggested he was clueless about his wife's business interests for 14 years and would not explain what the company did.

He insisted that his register was now fine and that he was the victim of a Labor Party smear campaign.

"As I become aware of my wife's commercial interests I declare them, as is appropriate," Mr Hockey said.

"My pecuniary interest register is entirely in order."

A spokesman for Special Minister of State Mark Dreyfus said: "The long-standing practice of parliamentarians' declarations of interests is an important element of our transparent democratic process."

Documents filed with the Australian Securities and Investments Commission show Mrs Babbage was appointed as a director of Steel Harbour Pty Ltd on April 2, 1998.

Her directorship includes the three years Mr Hockey was a member of the Howard ministry from 1998 to 2007 and spans more than some of the long careers of Mr Hockey's colleagues.

Mr Hockey was elected as the member for North Sydney in 1996.

But Mr Hockey's register of pecuniary interests shows he only put the interest on the public record on May 29 last year.

The move came as Mrs Babbage added a string of other directorships to her swag of corporate roles, including with Athletics Australia, the Australian Athletics Federation and Host City Marathon.

Mr Hockey said: "The Labor Party has previously engaged in this type of muckraking and then been forced to correct such unsubstantiated assertions. It is a desperate action from a desperate government."

MPs can be referred to a privileges committee inquiry over omissions with the interests register.

Punishments for knowingly providing incorrect information to parliament can include jail or fines.

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