The Modified Monash Model determines which towns can pay doctors extra money, and how much, through the GP Rural Incentives Program.

The model uses criteria such as remoteness and population size and has been in effect since July 1.

All but four Southern Highlands towns have been classified as 'MMM 3', meaning they have, or are within 15km by road of, a population of 15,000 to 50,000.

Hill Top and Colo Vale are 'MMM 3', as are Bowral, Mittagong and Moss Vale, a rating which affords doctors up to $12,000 extra pay over five years.

Bundanoon, Wingello and Penrose are rated 'MMM 5', which allows doctors up to $23,000 extra pay to practice remotely over the same period.

Robertson, a 'MMM2', has been excluded from eligibility to pay rural incentives, as reported in the Southern Highland News on July 3 (page 4).

"As Colo Vale and Hill Top are within 15km road distance of Bowral-Mittagong, a community with more than 15,000 people, these towns are classified as '3'," said a federal Department of Health spokeswoman, quoting "calculations provided by the Australian Population and Migration Research Centre".

Online map software estimates Hill Top (MMM3) is exactly 15km by road to Mittagong, the nearest CBD; Bundanoon (MMM5) is 17.3km to Moss Vale.

Local GP Dr Vincent Roche said there was "socio-economic inequality" in the provision of Southern Highlands medical services.

"The further out you go, the poorer they are ... and the harder it is for them to get [medical] services," Dr Roche said.

Neither Hill Top nor Colo Vale has a railway station, and the bus timetable is infrequent.

Bowral was "relatively over-doctored by conventional ratios," but, away from Mt Gibraltar, doctor numbers "run down ... out to Hill Top [and] Colo Vale," Dr Roche said.

"It's the same problem the council talks about with their swimming pools and libraries and the like.

"We are a shire where we don't have a concentration of population in one spot.

"But health care is very Bowral-centric and it would be nice to overcome that."