The nation's most senior financial officials have briefed both major parties on the consequences of a British exit from the European Union as Labor and the Coalition made a pitch to voters to stick with them in the event of an exit.
Reserve Bank of Australia governor Glenn Stevens, Treasury secretary John Fraser and Australian Prudential Regulation Authority (APRA) chairman Wayne Byers have briefed the parties under the caretaker conventions of the election campaign. It is understood they have said that an exit could unleash market swings in Europe with the risk of contagion spreading to other markets, including in Australia. 
Shortly before Britons started voting on Thursday, Prime Minister Malcolm Turnbull claimed only the Coalition would have the ability to steer the nation during what would be a global economic shock.
"It would be a very big shock if Britain votes to leave the European Union. There will be obviously great efforts to ensure that the consequences of that shock are minimised," Mr Turnbull said.
"There are a lot of headwinds in the global economy. There are a lot of challenges. There are a lot of things that are beyond our control that affect global markets that affect us."
In Britain, on the last full day of campaigning before the vote, both sides of the often-bitter debate had their final say ahead of what is expected to be a tight count. The pound hit its highest levels of the year on speculation that the Remain vote would triumph, fetching $US1.47 - up 0.6 per cent against the dollar on Thursday.
Queues formed at foreign exchange bureaux across London as people cashed in sterling ahead of the vote.
Australian markets ground higher on Thursday as nervous investors awaited the results of the referendum. The sharemarket's benchmark S&P/ASX 200 Index rose 0.3 per cent to 5280 points while the Australian dollar remained above US75Â¢.
The two final polls released in Britain hours before voting for the historic referendum began gave the Leave camp a slight edge as Britain was warned by European leaders that "out is out".
Financial and betting markets continue to put the likelihood of a Remain vote at close to 75 per cent. However, with the publication of opinion polls banned until voting closes at 8pm, two final surveys gave Leave a marginal lead.
The Remain camp, led by Prime Minister David Cameron, has been quietly confident that an improved showing in the polls over the past week would be enough to ensure a narrow victory. However, these final polls mean undecided voters will determine a result that sits on a knife edge.
Most economists predict the UK economy will suffer in the event of a Brexit. And officials based in the EU capital, Brussels, made it clear any decision to leave would be final, with European Commission president Jean-Claude Juncker reiterating on Thursday that "out is out".
Arguing that Mr Cameron had secured the "maximum" amount of concessions from Brussels prior to the referendum campaign, he added there would be no renegotiations.
"British voters have to know there will be no kind of any negotiation. We have concluded a deal with the prime minister," Mr Juncker said. "He got the maximum he could receive, and we gave the maximum we could give, so there will be no kind of renegotiation."
Mr Cameron echoed those words while addressing a Remain rally in Birmingham. "There is no going back," he said. "This is the choice of a lifetime. If you want to boil it down to one word it would be 'together'."