An extra 10,000 drivers challenged their parking fines last year - a 60 per cent rise - but for most their excuses failed to cut it, with only about 500 more tickets withdrawn than the year before.

And, unsurprisingly, the best chance of mercy from the Parking Review Office appeared to be buying a valid ticket in the first place.

Of the most common reasons Canberra drivers gave for failing to show a parking ticket, most were about claimed accidents in how the print-out was displayed, an Access Canberra spokesman said.

Tickets falling on the floor, being placed under a pile of old tickets or upside down, or being left in a drivers' pocket or wallet were among the leading excuses.

The spokesman said figures on the reasons which led to a parking fine being withdrawn were not accessible as they were not collated. However, he confirmed some drivers with validly purchased tickets not displayed correctly were among those ultimately spared from the frustrating penalty.

There were 104,046 parking infringement notices issued by ACT government officers in 2014-15, more than 16,000 extra than the year before when the rollout of 300 new parking machines was attributed as a disruption to enforcement. The rise was 19 per cent, but the figure was closer to the 100,099 in 2012-13.

However, the numbers of challenges was unique, as drivers lodged 27,967 written representations, up 62 per cent from 2013-14. It was also more than double the number received in 2012-13.

Yet only 7302 of these PINs were withdrawn, a rise of seven per cent from 2013-14. Withdrawals included fines issued incorrectly to drivers who paid using the Parkmobile phone app.

The ACT government increased the number of public inspectors by one-third last financial year, meaning another eight were on the beat.

Parking fine revenue was forecast to increase to $13.6 million in 2015-16, up about 30 per cent from the $10.5 million last financial year, which was $1.9 million less than expected.

The spokesman said parking officers were needed to ensure the safety of pedestrians and other motorists, protect public and business amenity and to provide incentives to ACT residents to use alternative forms of transport.

Most common reasons Canberra drivers give for failing to display a parking ticket: