The Perth man who smashed a lemon meringue pie in the face of Qantas chief executive Alan Joyce in protest against same-sex marriage has been fined $3,600.

Key points: Attack on Qantas CEO by Tony Overheu was "foolish" and "appalling", magistrate says

Attack on Qantas CEO by Tony Overheu was "foolish" and "appalling", magistrate says Overheu granted spent conviction after pleading guilty to assault, trespassing

Overheu granted spent conviction after pleading guilty to assault, trespassing Overheu's lawyer says his client has taken being banished from church "very hard"

Tony Overheu, 67, went uninvited to a business breakfast at a city hotel in May and shoved the pie in Mr Joyce's face while he was on stage addressing a crowd of about 500 people.

Overheu pleaded guilty in the Perth Magistrates Court on Friday to a charges of assault and trespass.

His lawyer Tom Percy told the court his client had borne no grudge against Mr Joyce.

But he said Overheu — a former farmer — had decided to attack him as "a stance against people in high places using their positions to make political statements".

In March, Mr Joyce was one of 20 chief executives of some of Australia's largest companies who signed a joint letter in support of marriage equality.

Some of Overheu's family have cut ties with him in the wake of the attack, the court heard. ( AAP: Richard Wainwright )

Mr Percy said Overheu had tried writing to newspapers and going on talkback radio but had not gotten much traction, so he made "a grave, foolish and significant error of judgement" to go to the breakfast.

He told the court his client had expressed significant remorse and had sent letters of apology to the organisations that were affected by his actions.

Mr Percy said Overheu had now been banished from his church — a move which he had taken "very hard" — and had been banned for life from flying with Qantas.

He also said some members of Overheu's family had "taken it badly" and cut ties with him.

In addition to the assault and trespass charges, Overheu pleaded guilty to damaging the lapel microphone Mr Joyce was wearing and admitted giving a false name to police after the incident.

Spent conviction for 'appalling' attack

Magistrate Greg Smith described the attack as "appalling" and said it was designed to humiliate Mr Joyce in front a large group of people.

"It would have caused shock, alarm and embarrassment to Mr Joyce," he said.

"It was planned in advance simply because Mr Overheu has different views to those held by Mr Joyce and many other people as well."

However, Mr Smith accepted Overheu was remorseful and said he had "already paid quite a high price" for a "foolish" act.

"You've attracted widespread condemnation and public ridicule, and even members of your own family have severed ties with you," he said.

Mr Smith granted Overheu a spent conviction, meaning he will not have a criminal record and will not have to disclose the offences to anyone.

In addition to the fine, Overheu was ordered to pay $269 in compensation for the lapel microphone and $188 in costs.

Mr Percy said his client had had "physical and personal difficulties" in recent years, including mental health issues.