A spike in family violence offences committed by women is one of the reasons Tasmania's women's prison needs to be expanded, the State Government says.

Key points: State Government to spend $4.7m for 20 extra beds at women's prison

State Government to spend $4.7m for 20 extra beds at women's prison Union welcomed the boost but urged more funding for prison workers

Union welcomed the boost but urged more funding for prison workers Attorney-General says there has been an increase in family violence offences by women

The Government said it would spend $4.7 million for 20 extra beds in the Mary Hutchinson women's prison as part of Thursday's state budget.

The Government will also commit almost $700,000 towards the Ron Barwick Minimum Security Prison.

Sarah Lovell from United Voice welcomed the funding, but said prison workers needed more assistance to cope with the changes.

"We would hope that the Government is matching any increase in funding that will increase capacity, with an increase in staffing to match that," she said.

Mary Hutchinson is Tasmania's only women's prison and has the capacity for just 46 female inmates.

A Department of Justice spokesman said the average number of prisoners was 43.

But the union estimated up to 52 women had been behind bars at the facility for months.

"The Mary Hutchinson Women's prison has been operating at either full capacity or over capacity for at least the last six to eight months," Ms Lovell said.

Drug use makes violent offending more likely: Goodwin

Tasmania's Attorney-General Vanessa Goodwin said at last count the prison system was sitting at 87 per cent capacity overall.

Ms Goodwin said one of the reasons for Tasmania's female prison being full to capacity was because of a spike in female family violence offenders.

"We talk quite a bit in the family violence context about male perpetrators of family violence, but there are also some female perpetrators of family violence," she said.

"Some of those are ending up in the prison system.

"Drug usage, potentially the involvement of ice, changes their offending profile so they are more likely to be involved in offences of violence."