Otago University law professor Andrew Geddis said Mallard was exercising general law in the issue of the trespass notices.
However, speaking to Newstalk ZB's Mike Hosking, Geddis also said that because the trespass notice has been issued for Parliament, an extra layer is added that states it can only be used in a reasonable manner.
"So, he'll [Peters' lawyer] argue that the decision to trespass is an unreasonable one given that he had a fairly low-level involvement in the protest," he said.
Peters confirmed he has sought legal counsel.
Geddis said the criteria used in the decision-making of issuing the trespass notice to Peters would be questioned.
"Also, there's a proportionality question – was Peters' actions in visiting the protest for that limited amount of time really serious enough to justify this decision and basically say to him you're not allowed to come to what is an essence of the heart of New Zealand's democracy."
Peters said he attended the protest as he felt the majority of protesters needed someone to listen to them.
"A lot of people had their lives utterly ruined. Many were not violent, many were not disruptive criminals.
"MPs refused to hear them and when someone went down and said 'I'll hear you' the Speaker has the audacity to trespass that person of Parliament."
Peters accused Mallard of dictatorial behaviour in line with a "banana republic".
He disputed Mallard's claim that as Speaker of the House his power is "pretty much absolute".
"No they are not Mr Speaker – you are constrained to the laws of this country like the rest of us."
Earlier, Peters had said there was a difference between those who were on parliamentary grounds taking an active part in the protest and those who were not.
"Remember the protesters asked me to come and speak with them - as they had asked every single current member of Parliament who had refused to."
