The Australian dollar has tanked and around $50 billion in shares has been wiped out on the local market after results showed Britain choosing to leave the European Union.

Results were showing a 4 per cent margin for the Leave camp.

Investors fled in droves after the news, with the ASX 200 extending losses to close 3.2 per cent lower to around two-and-a-half-month lows of 5,113, while the All Ordinaries slumped 3.1 per cent to 5,193.

The Australian dollar lost 3 per cent to 73.8 US cents.

The market earlier started to sell off when the Leave campaign overtook Remain, with high volatility as investors faced uncertainty.

"The surprise Leave victory in the UK's Brexit referendum has changed the calculus of global markets," IG market analyst Angus Nicholson said.

"Speculation in the markets will increasingly focus on the weakest links in the EU, those who are most likely to leave and those who would suffer most under the breakup of the EU."

"It is also very uncertain what this will mean for Britain and what sort of timeframe Britain's exit from the EU will take."

Red, red, red

Every sector on the local market was in the red, except for gold companies as investors eyed the safe haven metal.

Gold jumped 5 per cent to $US1,318.09 an ounce on the Brexit news.

All the big four banks sank by more than 3 per cent, while blue chip miners BHP Billiton and Rio Tinto slumped 7.9 per cent and 6.5 per cent respectively.

Gold miners Newcrest Mining and Regis Resources gained 8.8 per cent and 8 per cent each, supported by a stronger gold price.

A number of Australian companies that derive a proportion of their revenue from the UK also dived. They included Amcor, which lost 2.7 per cent, Domino's Pizza, down 3.6 per cent, and News Corp, which lost 1.7 per cent.

Asian markets also tumbled into afternoon trade, with China's Shanghai Composite dumping 1.3 per cent to 2,854.1, while in Japan, the Nikkei dived 7.9 per cent to 14,952.

Asian currencies also suffered on worries investors would pull funds from emerging markets.

British pound slumps to lowest level since 1985

The British pound slumped over after a Leave result in the Sunderland electorate came in stronger than expected, extending falls as the BBC forecast a win for Leave.

The pound shed more than 10 per cent of its value to $US1.346 — its lowest level since 1985 — before recouping some losses.

Sorry, this video has expired ABC Business Editor Ian Verrender says the result has been a shock on global markets

Traders experienced one of the busiest 30-minute periods between 9:00 and 9:30 pm British standard time.

"We have seen the biggest intra-day volatility in the pound on record, as evidenced by it hitting a yearly high and yearly low within hours of each other," Mr Nicholson said.

By 4:31pm (AEST), the pound was down 7.8 per cent, in its worst session on record, to $US1.37.

The Australian dollar meanwhile shot 5.2 per cent higher against the pound to buy 53.8 British pence.

Bloomberg price chart showing the pound's decline as votes are counted.

Futures trade points to more carnage

Futures trading indicated more carnage to follow when the London stock exchange and Wall Street open later in the global day.

The EMINI S&P 500 Futures lost 5 per cent, while the Dow Jones Index Futures was down 4 per cent.

In London, the FTSE 100 is expected to slump when it opens, with FTSE futures indicating a loss of 8.8 per cent or 555 points.

"The process of leaving the EU is unprecedented and will be drawn out and complicated," said Savanth Sebastian, economist at CommSec.

"It is unlikely that EU members are going to make it easy for the UK to leave, in effect setting a precedent to ensure that other countries think twice about revoking EU membership."