Gavin Preston and the men hugged and shook hands in the outdoor compound of the Eucalypt unit before they all walked to a grassed area and a picnic table and sat down, began talking in the sun and were joined by a sixth inmate.

But a Melbourne court has heard how the show of friendship abruptly turned to a "frenzied" attack with "shivs", spikes and blades unleashed in an attack on Preston by members of the jailhouse Prisoner of War (POW) gang deemed "the law of the jungle".

Magistrate Ross Maxted was told that after the attack a letter to Matthew Johnson - the murderer of gangland boss Carl Williams - was found in Preston's cell that indicated there had been a previous incident between Preston and another member of the POW.

Senior Crown prosecutor Peter Rose QC told Melbourne Magistrates Court on Thursday that after Bradley Thompson first struck Preston from behind, the others joined in, forcing Preston backwards into a raised garden bed where the assault continued until prison officers arrived.

Mr Rose said that Preston received nine stab wounds, multiple lacerations to his face and head and a 15 centimetre laceration down his right jawline, but still managed to kick one of his attackers twice in the head.

Mr Thompson, 25, Lucas David Milne and Jason Anthony Evans, both 34, and Aaron Phillip Fenton and Matthew William Lovitt, both 37, have pleaded not guilty to charges of intentionally causing injury and affray on July 28 last year.

A sixth accused, who reserved his plea, was earlier sent for trial.

Preston was jailed in August for 11 years with a minimum of nine years after pleading guilty to defensive homicide over shooting a drug dealer in the head and to shooting and seriously injuring a second victim in 2012.

He is suspected of having earlier shot and seriously wounded former Bandidos bikie enforcer Toby Mitchell outside a Brunswick gym.

Mr Rose told Mr Maxted on Wednesday in his opening to a contested committal hearing that Preston had refused to assist police, consent to his injuries being photographed or releasing his medical records.

A prison officer told barrister Patrick Casey, for Mr Thompson, that she had concerns for Preston's safety because of his "profile in the prison system" and that that day was his first in "mainstream" in Barwon after transfer from another facility.

Mr Rose later announced that after accessing Preston's subpoenaed medical report, he concluded that it did not support a charge of intentionally causing serious injury and a related one the men faced.

On Thursday morning Mr Rose and barristers for all accused told Mr Maxted that by agreement - but subject to his consent - all charges would be replaced by a single count of intentionally causing injury, to which they would plead guilty, and that it be dealt with by him.

Mr Maxted was told that such a course would avoid the need for a lengthy trial or the committal continuing, the calling of further witnesses and so would save the community related costs and expenses.

Mr Rose also produced an agreed summary of the offence that stated a prison "code blue" was called when the incident began and that officers ran to the scene to stop the assault.

As two alleged assailants ran and jumped into a swimming pool, another broke free of guards to join his alleged accomplices in continuing the assault on Preston who was treated by medical staff and hospitalised.

In refusing the defence application for summary jurisdiction, Mr Maxted said while "helpful", he was not bound by their submissions and remarked that if he was an accountant they would be "extremely interesting propositions".

He said costs were just one of the serious matters he had to consider, but they were not the "determinative" issue as he also had to consider others such as the seriousness and nature of the offences, the manner they were committed, the degree of organisation and any aggravating circumstances.

Mr Maxted decided, given the discount for their guilty pleas, that the maximum two-year maximum jail term available to him for that charge - it is 10 years before a higher court - was not "sufficiently broad enough".

He also noted that any prison environment needed to be safe and that prisoners ordinarily would be safe rather than subjected to the "law of the jungle" where others could "extract revenge".

General deterrence, Mr Maxted said, was the most significant factor in the sentencing outcome.

After his decision, Mr Maxted struck out the two more serious charges and each man then pleaded not guilty and was sent for trial in the County Court.