The minimum topweight in benchmark races will be raised two kilograms from October following a Racing NSW review into Sydney field sizes.

Racing NSW chief executive Peter V'landys said the compression of weights had become an issue and hoped increasing the minimum topweight to 61kg would provide more opportunities for lower-rated horses to progress through the grades.

"The increase in the benchmark weight and overall spread of weights will make for more competitive races and improve winning opportunities for more horses at the bottom of weights," V'landys said.

The change to minimum topweights was among several amendments to the handicapping system announced by Racing NSW on Thursday.

Handicappers have also been given more discretion in the way they weight a horse against its benchmark rating.

Country-performed horses can currently be handicapped up to 3kg below their benchmark rating when they tackle a metropolitan race but Racing NSW's review found that was insufficient.

Under the changes, bush gallopers will be given up to 6kg less than their benchmark level in a bid to help more country-trained horses make a successful transition to city racing.

Handicappers will also have more room to move when determining whether to decrease the benchmark rating of a horse struggling for form.

Apprentice jockeys will also benefit and will be able to ride more winners before their allowances are reduced.

However, the biggest change will be for junior riders who finish their time.

They will be allowed to continue with a 1kg allowance during their first season as a jockey until they reach 90 winners to help their transition to the senior ranks.

The changes will be introduced from October 1 this year and will be reviewed again in six months.