HEY Dad! actor Robert Hughes has been bailed after appearing in a London court after a two-year international investigation into alleged sexual and indecent assault charges against five girls - including former cast members.

Former Hey Dad! co-star Sarah Monahan - who played Hughes' youngest on-screen daughter, Jenny - last night tweeted she was "crying with happiness right now", following the news of Hughes' arrest.

Ms Monahan was woken at 2am at her home in the US yesterday by a text message from detectives to tell her that Hughes had been arrested.

"Wow, it's 3am here and my twitter box is exploding," she wrote on Twitter.

"Well I got the call at 2am my time, promised I wouldn't leak it out till they made their official announcement."

"Thanks to the team at Strike Force Ruskin. Lots of hard work by all. You guys rock.''

Sex Crimes Squad Commander Detective Superintendent John Kerlatec said police spoke with more than 200 people during the investigation that led to the arrest of Hughes.

"Sexual assault leaves enormous scars on people,'' he said.

He said the alleged victims had been informed of the arrest.

If found guilty, Hughes could face up to 10 years in jail, Supt Kerlatec said.

"We're talking about sexual assaults against children," he said.

"That's a very serious crime ... that's a crime we take very seriously."

Detective Inspector Kerlatec said NSW Police had worked extensively with the Australian Federal Police and London Metropolitan Police to arrest Hughes yesterday.

Hughes appears in London court

Hughes, 63, once Australia's favourite on-screen father as Martin Kelly, was taken into custody at his London residence about 7am local time (4pm AEST). He was then released on bail, with a court appearance scheduled for September.

A police source last night said Hughes was "shocked" when arrested by London Metropolitan Police and taken to Belgravia Police Station to be charged with the sex offences. Last night, looking dishevelled with a scruffy grey beard and blue business shirt, Hughes briefly fronted Westminster Magistrates Court.

He confirmed his name and age to the judge before the case was adjourned until later in the day.

Hughes then re-appeared for a short hearing, during which the prosecution argued he was a flight risk.

Hughes was granted bail but he must wear an electronic anklet and stay away from children under the age of 16. The charges relate to alleged offences against five girls in suburbs in Sydney's north.

The court heard Hughes had allegedly sexually assaulted a girl who was five or six years old at the time.

He also allegedly kissed a 15-year old girl and made her touch him through his clothing, the court heard.

The prosecution also alleged Hughes indecently exposed himself to and indecently assaulted a nine-year-old girl; sexually assaulted a 15-year-old girl and exposed his penis to a cast member of Hey Dad! who was 12 or 13 at the time.

Australian detectives will now fly to London to apply for Hughes' extradition. The hearing will be on September 19.

With 45 days to appeal any extradition order, it could take several months before Mr Hughes is returned to Australia.

How the investigation unfolded

It has been a two-year investigation by eight members of the NSW Police sex crimes squad, which established Strike Force Ruskin in 2010 in response to Monahan's official police complaint.

At the time Actress Simone Buchanan, who played Debbie in Hey Dad!, also spoke out, claiming she too had been touched inappropriately by Hughes.

As the allegations against Hughes became public, another four alleged victims were identified and officers interviewed more than 100 witnesses across four countries.

The inquiry was quickly broadened to include previous claims of abuse stretching as far back as 1982, when a teenage school friend of a member of Hughes' family made a complaint to police.

According to one victim, that complaint was "swept under the carpet'' at the time -- the paperwork was also lost over the decades that followed, which set back the Ruskin investigation.

In October 2010, that alleged victim travelled to Sydney from Brisbane to be re-interviewed about the matter by officers

Other friends of the Hughes' family member were also interviewed at length over the lifespan of the Ruskin investigation.

Two months ago detectives travelled to Melbourne to interview a journalist who had spoken to some of the original cast members.

Detectives travelled several times across the world to gather statements and meet with witnesses.

In September 2010, they went to four cities in the US, including Los Angeles and New York where they took statements from stars who had acted on the show early in their careers. Ms Monahan was also interviewed by the strike force detectives in Texas.

The investigation's funding was questioned at several points by senior police. In mid-last year, the police brief of evidence, which contains more than 100 witness statements, was handed to the Director of Public Prosecutions for their consideration. From there, taskforce officers waited for the determination.

It took months to return to police with advice on whether they could reasonably press charges. A month ago the DPP ruled police had enough evidence to proceed with 11 charges and an arrest warrant was issued for Hughes.

The Daily Telegraph has been told that police sought more charges against Hughes, however the DPP determined there was not enough evidence to prove them in court.

"There were a number of charges where there wasn't sufficient evidence,'' a senior investigator said.

NSW officers then contacted the Australian Federal Police and the London Metropolitan Police in a bid to locate Hughes, finding him in London.

"He spends a lot of time between the UK and Singapore -- he travels between them quite regularly,'' an officer said.

"It wasn't overly difficult to locate him."

- with AAP