The new cells will be bigger than the old, at 11 square metres each, instead of eight square metres, which is the size of the old cells and the standard nationwide, and will allow prisoners to be separated more effectively. Prisoners will be held in smaller groups - up to 20 in each separate wing of the new blocks.

Corrections Minister Shane Rattenbury, who toured the site on Tuesday where earthworks have begun, said the prison had been under pressure since a big spike in detainee numbers in 2013, when the prison population had increased by 100. There were now between 320 and 340 prisoners behind bars, with a capacity of 370. The new blocks would increase capacity to 480, with room for more if prisoners were kept two to a room.

Mr Rattenbury said the high number put constant pressure on staff trying to separate prisoners of different security classifications and different needs, and prisoners who needed to be separated because of previous connections in the community. This was a particular problem in Canberra because of the small jurisdiction and only having one prison.

Capital works director in the justice department Greg Hammond said one of the two new blocks was a special care centre with space for rehabilitation programs for sex offenders and people with drug and alcohol problems. Thirty people could be held in the special care unit, in separated wings of 10 each.

The other was a 56-cell, two-storey unit, with 14 cells in each wing. It would house 80 prisoners, with some doubling up, and each cell, like others in the prison, had access to natural light and fresh air. Each wing of the new blocks had a wheelchair accessible cell, of 13 square metres.

In other parts of the prison, up to 48 people are housed in blocks of 26 cells, making it difficult to keep groups separate.

Manger of custodial operations Don Taylor said the new, smaller blocks, would make it easier to manage the prisoners.

"It's not as easy to find 48 people that can live together as it it is to find 20," he said.

The expansion is being fast-tracked, with the first block to open next year and the second in 2016.

The Alexander Maconochie Centre is only six years old, and concerns were raised while it was still being planned that it would not be big enough. Mr Rattenbury said he had not been part of cabinet at the time, but it was "notoriously difficult" to model prison numbers, and jurisdictions around the country were facing similar pressures with increasing numbers of prisoners.