It might be one of Auckland's most iconic events but traffic restrictions will mean not everyone gets a chance to take part in the city's marathon today.

More than 10,000 people will participate in the full variety of classes - from 5km to full marathon - but Transit New Zealand restrictions mean that demand far exceeds supply.

The half and full marathon classes sold out in May.

Since then the Herald on Sunday has found that hundreds of people - an estimated 10 per cent of the total who will finish both races - have been forced to pay a premium to enter the event. They have had to buy entries from Trade Me at inflated prices.

Some, desperate for the experience of running over the Harbour Bridge, forked out $180. The official half-marathon entry fee is $95.

Second-hand entries also attract up to $50 in transfer fees, to change names and other such details of the participant.

Auckland Marathon organiser Scott Newman admits the idea that scalpers have entered the market is disappointing but says there is little that can be done.

The field has been increased over the past two years but concerns about the volume of foot traffic on the bridge will always limit the final number.

"Safety concerns are paramount for us," says Newman. "We would like to increase the numbers but it would require significant city support and is out of our hands."

Transit NZ opens just one lane to runners and seems unwilling to allow more, fearful of the impact on traffic and the stability of the bridge.

In Sydney, the entire eight lanes of its bridge are available during its marathon, though they do have a tunnel that spans the harbour underneath the bridge.

An idea to start the run closer to the bridge - perhaps at Northcote or Birkenhead - was dismissed by Newman.

He admits it would mean runners would arrive at the bridge faster and cut down the amount of time that lanes would have to be open but it is not seen as a feasible option as the intensity of volume of runners could cause structural concerns.

Perhaps the free market may have the last say. In the past month, scalpers have been outfoxed as prices for second-hand entries have dropped below the entry price.

Some last-minute entries have gone for as little as $50, so those scalpers have actually lost money.