At about 10am Wednesday, 200 disgruntled police, corrections and army officers gathered at Unagi Oval to protest about their salary deductions, and more soldiers from two barracks arrived to join them soon after.
The deductions over the last fortnight range between US$26 and US$80 (K100 and K300).
The Assistant Police Commissioner Anthony Wagambie addressed the protesters.
"Frustrations boiled over so they got into their vehicles and stormed parliament...they opened the gates and went into parliament," RNZ Pacific's Papua New Guinea correspondent Scott Waide said.
"There was no real resistance to stop them...it was a rowdy crowd, the defence minister had attempted to speak to them outside of parliament before they walked in," he said.
He said he has been told by government that the deductions are the result of tax glitches.
"They are currently at parliament right now. I think somebody is addressing them, I'm not sure to sure whom as I've just seen the videos.
"There's a large group of police, army and correctional services personnel at parliament house right now, protesting over the deductions.
"The public is relatively quiet, nobody from the public has joined the protest in large numbers. Everything has remained calm."
He said police personnel include units from Sector Patrol, Special Services Division, Water Police and Dog unit, and army personnel are also present.
During the protest, Police Association president Lowa Tambua demanded an answer as to why there had been deductions.
"We want an immediate answer from the Minister of Police and the Prime Minister," Tambua said.
"We we're all caught by surprise...come and address my members as to why this has happened.
"Don't hide between the parliament house...come over here and address our police men and women."
The Internal Revenue Commission (IRC) has said there are no tax increases in the 2024 budget.
According to the IRC commissioner-general Sam Koim "there has been no tax increase" to their salaries.
In a short statement, Koim said: "There was a technical glitch on the Alesco payroll configurations and hence the deductions."
'Instructions issued'
According to PNG Bulletin, Prime Minister James Marape reiterated the government's stance on tax, emphasising that there have been no new tax increases.
"Addressing concerns raised by security personnel regarding taxes, Marape assured that actions are being taken to investigate the source of deductions," Marape said according to PNG Bulletin's report.
"He informed that instructions have been issued to the finance department to rectify the issue promptly.
"Marape assured genuine police officers that their concerns are acknowledged, and advice will be provided later in the day," the news website reported.