The magistrate guidelines for a first time offender pleading guilty are for a custodial sentence.
However with prison places at a primium there is pressure not to send people to prison unless necessary.
In short it will probably depend a lot on your particular case. For example whether it's seen as a single out of character act and you're seen as of little risk of reoffending.
Mitigating factors include:
Minor injury, provocation, single blow
Agravating factors include:
Kicking or biting, premeditation, a weapon being used, victim being vulnerable, domestic setting.
Mitigating factors include evidence of remorse, cooperation with the police voluntary compensation.
I wouldn't think you'd go to an open prison, that's usually bankers and accountants not violent criminals.
It's likely to be pretty unpleasant, overcrowded and pi$$ing in a bucket. Media stories of breakfast in bed are a bit of an exageration.
I suggest you do all you can to avoid it, genuine remores would be a real good start.