MPs taking advantage of the taxpayer-funded travel perk to take partners abroad could earn free flights on trips that cost them next to nothing in the first place.

The Parliamentary Service - the organisation responsible for administering politicians' travel perks - this week confirmed to the Weekend Herald members receive free membership to Air New Zealand and Qantas air points programmes.

Air New Zealand has a travel office in Parliament.

While MPs are free to choose their preferred carrier, Air New Zealand is the most frequently used, at least for domestic travel.

The air points programmes are between the individual and the airline or sponsor organisation - such as a bank credit card - and benefits are earned from personal expenditure and travel.

A spokesman for the Parliamentary Service confirmed members of Parliament were "strongly encouraged" to use any air points earned from publicly funded sources to offset travel costs - "both domestic and international".

The air points schemes could mean hefty benefits for Act Party leader Rodney Hide and his partner Louise Crome after their taxpayer-funded trip to London, Vancouver and Los Angeles.

Though Mr Hide's trip was paid by Ministerial Services, he dipped into his MP's travel perk - which allows for a 90 per cent subsidy - to pay for Ms Crome to accompany him.

Though Ministerial Services considers it "not inappropriate" for staff to use ministers' air points "to offset either internal or overseas taxpayer-funded travel", there is no such requirement from the Parliamentary Service.

A check of Air New Zealand's website shows - if they flew the national carrier - Ms Crome's business class flights would have earned about 985 air points dollars, enough for a return trip to Tahiti or Noumea, or a one-way ticket to most Australian and Pacific Island destinations.

As revealed in the Weekend Herald today, the couple also holidayed in Hawaii, at a cost of $10,000 to the public purse. That trip would, flying business class, have earned the couple another 840 air points dollars, sufficient for a one-way fare to most Australian destinations.

The taxpayer paid more than $25,000 in travel perk subsidies for the couple's flights, and some domestic travel for Ms Crome.

Others who could earn free flights from air points include Agriculture Minister David Carter, whose travel perk paid for wife Heather to accompany him to London and Brussels, at a cost to the taxpayer of $11,717.

Return business-class flights to London on Air New Zealand would have realised 740 air point dollars - enough for a one-way ticket to most Australian and Pacific destinations.

United Future leader Peter Dunne confirmed his wife accompanied him to Europe at a cost of about $12,000 to the taxpayer.

Prime Minister John Key is comfortable with ministers taking partners on overseas trips, as long as they paid for the travel themselves, and using the perk met the definition.