He was denied police bail as a matter of course, under a policy aimed at tackling domestic violence, but Dr Bain said that was not explained to him and custody officers who phoned four lawyers on his behalf were stymied by answerphone messages.
The coroner said the police acknowledged at the start of his inquest in January that procedural lapses created a circumstance in which Mr McGuire was able to take his own life.
He praised the police for the "frankness and honesty" of their own investigation into Mr McGuire's death, and said he was satisfied after viewing a new state-of-the-art custodial facility that there were now sufficient practices and procedures in place at Rotorua to prevent similar tragedies.
But he was concerned to discover similarities between the procedural failings in Mr McGuire's case, and those surrounding the death in 2002 of an intoxicated man with head injuries who was held in custody in a Rotorua police cell for 32 hours without being assessed or monitored properly.
Although the police were said at an inquest into that death to have made significant procedural improvements, Dr Bain said they needed to be "respectfully reminded in respect of both these cases that they must constantly review their procedures and not only have them in place but ensure that they are being followed".
Dr Bain has recommended that the Legal Services Agency urgently establish a national on-call service, available at any time of day or night to prisoners in custody.
An 0800 number or some similar system "would at least mean they could have explained to them what is happening to them and that they will almost certainly, in a case such as Mr McGuire's, get bail the next day".
He said that, having viewed the cell in which Mr McGuire was placed, it was "not hard to see how one would become very, very depressed if that was your first time in custody - it would be very forbidding indeed".
It was incumbent on the Legal Services Agency to provide enough resources for a national phone service, and to make sure the lawyers were paid properly, as people could notbe held without access to legaladvice.