'World has changed': One Nation declares victory in WA

One Nation's Luke Herdegen had secured 57 per cent of the two-candidate preferred vote when counting closed for the day with nearly 75 per cent of ballots counted.

It is the first seat the right-wing party has claimed in WA's lower house.

One Nation federal MP Barnaby Joyce said the party wasn't getting ahead of itself, but the by-election win sent a strong message to Canberra.

"The world's changed, hasn't it, politics has changed," he told News24 on Sunday.

"What we've seen there is quite a number of people who basically would have been former Labor Party voters ... they find it easy to make the jump to One Nation."

Mr Herdegen had secured 39.1 per cent of first preference results as of Saturday night, ahead of Ms Tree's 24.4 per cent and Liberal candidate Ryan Robertson's 17.3 per cent share.

WA One Nation leader Rod Caddies noted the historic moment as he declared "the two-party system is dead".

"You're witnessing a result they said was impossible," he said.

"Labor have owned this part of Perth. But a safe seat does not stay safe if you ignore it long enough. 

"Tonight Labor found out the hard way."

Mr Joyce said traditional Labor-held seats could no longer be taken for granted.

"If you think 'oh, these people will just vote for me', then that's a sense of arrogance and arrogance will get your political hide delivered to you on a plate," he said.

One Nation's performance in WA has been closely watched across the nation, particularly in Victoria and NSW, where state elections are due in November and March respectively.

On the ground in Secret Harbour, Labor was on the nose with many voters as cost-of-living pressures, housing shortages and a perceived lack of community amenities topped the list of concerns.

Local Luke Pors voted for One Nation, believing the minor party would do a better job than either Labor or the Liberals.

"We know what their true goal is and I think just in One Nation we trust," he said.

The Qantas flight attendant was also impressed by Mr Herdegen's campaign.

Federal Labor frontbencher Pat Conroy said the result was a tough outcome for the state government of Roger Cook.

"Premier Cook and his team will be analysing the result and drawing the lessons from it that they need to," he said.

"I'd be hesitant to draw lessons from a single state by-election and try and apply that to national politics."

Opposition home affairs spokesman Johnathon Duniam said the Liberals should not have admitted the by-election would be a challenge for the party before it even took place.

"When we start talking about conceding or coming in second or finding ways to work with other people before we've even run the race, then yeah, people are not going to back us," he said.

"We've got to fight hard and we've got to give people a reason to vote for us."

The WA result follows One Nation securing four lower-house seats in South Australia's March state election, the first lower-house places the party had ever secured outside of Queensland.

The anti-immigration party also won a maiden lower-house seat in the federal parliament in May when it triumphed in the Farrer by-election.