The Speaker is investigating whether former MP Garry Knapp is eligible for a perk for international travel that he has claimed for almost three decades - but it appears Mr Knapp was eligible for it when he left Parliament in 1987.

The rebates for international travel are currently only available for ex-MPs who served for three or more terms of Parliament and were elected before 1999.

Mr Knapp was an MP from 1980 to 1987 - two and a part terms.

A spokeswoman for the Speaker confirmed he had asked Parliamentary Service for advice on Mr Knapp's eligibility.

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However, an old letter from former Speaker Sir Doug Kidd setting out the history of the perk shows that in 1987, when Mr Knapp left Parliament, he was entitled to a 50 per cent rebate on international travel.

The rules were tightened in 1989 so the rebate only applied to MPs who had served more than three terms. Only MPs who had been a Speaker or Minister were eligible after two and a part terms.

It is unclear whether that change was intended to apply retrospectively - which would have stripped Mr Knapp of his entitlement.

The letter from Sir Doug was to former Act leader Rodney Hide in 1997 and shows how the perk came into being in 1972 and the changes made over the year, such as including spouses.

Mr Knapp claimed about $3000 over the last year including $2800 for international travel.

Mr Knapp was a Social Credit Party MP who entered Parliament in the 1980 bye-lection for East Coast Bays, beating National's candidate Don Brash. He changed his allegiance to the Democrats in 1985 and was defeated in East Coast Bays by Murray McCully in the 1987 election.