Warnings about the vulnerability of Christchurch's heritage buildings in an earthquake were given 15 years ago on television.

The earthquake dangers facing Christchurch were examined in an Inside New Zealand documentary broadcast on TV3 in 1996.

A clip from the documentary posted on YouTube on March 5 had received more than 3000 hits by yesterday.

The TV3 documentary described Christchurch as having "a fatal flaw."

"It sits on a soft, shaky sponge of river stones and silt half a kilometre deep. In any decent quake, Christchurch will shake like a leaf," it said.

Christchurch was described as having "quite a high susceptability to liquefaction", especially in eastern areas.

Concern was also raised about the "hundreds" of old buildings that had not been strengthened.

The heritage list of old buildings "defined most of the earthquake-prone buildings" in Christchurch, one expert said.

Meanwhile, owners of earthquake-prone heritage buildings in the city were warned last year that they could have to pay at least $169 million for strengthening works - from 2012. The warning appeared in a story which ran in The Star in March, 2010.

Nearly 500 heritage buildings in Christchurch and on Banks Peninsula would have been required to be strengthened or demolished within 15 to 30 years under amendments proposed for the city council's 2006 Earthquake-Prone, Dangerous and Insanitary Buildings Policy.

According to a review of the policy, these 490 buildings were only a fraction of about 7658 buildings in Christchurch that would be likely to collapse in a "moderate earthquake" - the equivalent of an earthquake with a magnitude around 6.5 on the Richter scale. Last month's devastating quake in Christchurch measured 6.3.

Un-reinforced masonry buildings - such as 295 of the city's listed or scheduled heritage buildings - were considered particularly earthquake-prone, according to the story in The Star.

Christchurch Heritage Trust chairman Derek Anderson was quoted as saying that the scale of the problem was huge, with 150 heritage buildings located within the city's central business district.

"The council has no realistic policy for heritage buildings and they will collapse in an earthquake," he warned.

Scientists had predicted a 50 per cent chance of a magnitude 7 quake hitting Christchurch in the next 50 years.

"We're well overdue for a big one," Mr Anderson said last year.

The Christchurch Heritage Trust had also been involved in strengthening the Excelsior Hotel building at the corner of Manchester St and High St and the Star-Times building in Cathedral Square.

Mr Anderson believed there needed to be a partnership between the Government, the city council and building owners on earthquake strengthening.