Prime Minister Tony Abbott says he can't guarantee university fees won't double because of his government's deregulation of the sector.
But he can say they won't have to pay a cent up front.
A day after student protests against changes that allow universities to charge higher tuition fees, Mr Abbott could not assuage fears they may double as a result.
"There are lots of things that I can't guarantee, because we live in an uncertain world, but I can guarantee that no one will have to pay a cent upfront because there'll be these fee help loans to cover their up front costs," he said on Thursday.
Mr Abbott said increased competition in the market would mean some fees go up.
"(But) I suspect others may well put their fees down," he said.
Mr Abbott's comments come as the University of Sydney's vice-chancellor Michael Spence warned that fee deregulation risked pricing middle-class families out of a tertiary education.
"It's the ordinary Australians that I think aren't getting enough of a guernsey in this conversation," Dr Spence told Fairfax.
Universities have called on the government to take more time to investigate any unintended consequences of the higher education changes before setting them in law.
Two arrested in violent student protest
An activist was arrested for picking up a flare as thousands of angry federal budget protesters brought Sydney's CBD to a standstill.
The man, aged in his 20s, was forcefully bundled to the ground by police and put in handcuffs as thousands of protesters marched from inner city Ultimo to Town Hall on Wednesday afternoon.
The man picked up a lit orange flare off the ground and held it aloft before being swarmed by about six police officers, who pushed him against a wall and handcuffed him.
Hundreds of angry protesters surrounded the group and chanted "let him go, let him go" before mounted police moved in and dispersed the crowd.
The man was led away as protesters, mostly students from five different NSW universities, staged an impromptu sit-in at the junctions of George and Hay streets.
After about five minutes, they continued marching - surrounded by police - along George Street, bringing the CBD's main thoroughfare to a standstill.
They were protesting the federal government's budget cuts and fears that university prices will rocket under the Abbott government.
Students and their supporters had gathered earlier at the University of Technology Sydney in Ultimo, before marching through the city.
The group chanted "F*** you Tony Abbott, f*** you" and waved placards reading "Pyne you shameful grub" - a reference to Education Minister Christopher Pyne.
There was a person dressed as the Grim Reaper and wearing a Christopher Pyne mask, carrying a coffin with the words "our education" emblazoned on the side.
President of the National Union of Students Deanna Taylor said that the budget had been "cruel".
"It was a cruel, harsh, mean budget and it's going to absolutely savage higher education in Australia," she told the crowd.
"In one swoop on Tuesday, the federal government not only attacked the universal healthcare system as we know it, schools, pensioners, hospitals, they also attacked higher education and young people," Ms Taylor said.
NSW Greens Senator Lee Rhiannon has advocated, in some circumstances, for university campuses to be made no-go zones for conservative politicians.
It comes after Prime Minister Tony Abbott and Mr Pyne cancelled a visit to Victoria's Deakin University on Wednesday on the advice of police.
Foreign Minister Julie Bishop, Mr Pyne and ex-Liberal MP Sophie Mirabella have all been targeted by protesting university students since the budget was handed down.
A 23-year-old man, who police claim used a flare, was charged with possessing a dangerous article in a public place.
Police arrested another man who allegedly assaulted an officer.
He was charged with resist and arrest, hinder police and assaulting a police officer.
Both men are due to appear before Sydney's Downing Centre Local Court on June 3.
Police operation commander Simon Hardman said the majority of protesters abided by directions and made their way to Town Hall with little disruption to traffic.
"There will always be a handful of people who want to cause trouble but they were dealt with ensuring the impact on pedestrian and vehicular traffic in the CBD was minimised as much as possible," he said in a statement.
Meanwhile, scuffles broke out as protesters angered by federal budget cuts clashed with police on the steps of Parliament House during a similar rally in Melbourne yesterday.
Protesters charged at a police line as they reached the steps of Parliament on Wednesday afternoon.
There was a short scuffle, with officers holding back the demonstrators before they returned to peacefully protesting federal budget reforms.
After the rally broke up, a group of 20 people staged a sit-in at the intersection of Bourke and Spring Streets outside Parliament House, blocking trams and traffic.
They were ringed by dozens of uniformed and mounted police, who systematically removed the protesters one by one within an hour of the rally's end.
Demonstrators were warned twice to move along before groups of five or six police officers moved in, carrying them away from the intersection.
Among them were a few underage students, some aged 15 and 16 and dressed in private school uniforms.
At least 2000 people rallied outside the State Library in the protest, which was organised by the National Union of Students and the National Tertiary Education Union.
Many then marched along Swanston and Bourke streets to Parliament House, chanting slogans including "no cuts, no fees, no corporate universities" and "no ifs, no buts, no education cuts" as they marched.
A speaker at the rally said the protest would be the first in a series of "rallies, sit-ins and civil disobediences" as they vowed to fight the budget cuts.