The original Perth observatory was once isolated on a hill above the city, providing vital weather and bushfire warning information.

Now surrounded by various State Government offices, it has been preserved and is now home to the National Trust of WA.

The observatory dome was demolished in the 1960s to make way for the building of Dumas House for government offices but the former chief astronomer's house remains, and in 1999 the National Trust moved into the building and restored it to its former glory. Perth Observatory 1896-1900. ( Supplied: State Library of WA )

The observatory was a vital piece of infrastructure for Perth when it was completed in 1897, the National Trust's Graeme Bridge said.

"The then Premier Sir John Forrest wanted to establish a fully functioning observatory, so he petitioned parliament for land to be set aside and funding to build and equip it," Mr Bridge said.

"Once it was completed in 1897 the new astronomer William Earnest Cooke was appointed and his family moved into this house, which was part of a complex of three buildings.

"It was to establish and record weather patterns as they happened in Perth.

"There were astronomical telescopes, wet and dry bulb thermometers, rain gauges, anemometers for wind speed and seismographs."

The old anemometer (wind speed measurement device) on the old Perth Observatory house tower. Dumas house looms in the background. ( 720 ABC Perth: Emma Wynne )

The other vital function of the observatory was to help the residents of Perth to accurately set their clocks to western standard time.

"Prior to that they had been using sundials but it was inaccurate up to about 20 minutes," Mr Bridge explained.

The former ground floor parlour, now a meeting room at the National Trust office. ( 720 ABC Perth: Emma Wynne )

"The observatory used sun-tracking equipment in the transit room, and to tell the people of Perth what the time was there was a canon fired daily at 1:00pm so people could set their watches."

Observatory staff climbed the hill above St George's Terrace every day to work with the chief astronomer, William Earnest Cooke, but the family was quite isolated living on the hill by Kings Park, and regularly had to look out for bushfires coming from Kings Park.

"It would have been a lovely building for the family to live in because it was very extensive for its time, but it would have been very isolated," he said.

"They were up here on their own."

In 1904 the new Western Australian Parliament House opened a short way down the hill from the observatory.

Miss Finch reading a rain gauge at Perth Observatory as rain gives relief from a heat wave in 1950. ( Supplied: State Library of WA )

"In the 1920s the state government realised that the space in here could be used by government departments but the work at the observatory still continued until the 1960s," Mr Bridge said.

"In the 1960s, the government built Dumas House as government offices and in the course of construction they had to demolish the dome building and the transit room for the excavation works.

"All the equipment was transferred to a new observatory at Bickley."

The house itself also came close to demolition when the government made plans, never realised, to build a further five office towers like Dumas House in West Perth.

The roof of the old Perth Observatory house, viewed from the tower. ( 720 ABC Perth: Emma Wynne )

"Fortunately Ian Medcalfe, then a member of the Legislative Council, got a group together and petitioned the government not to demolish it so we can enjoy it today," he said.

The National Trust moved in in 1999 and has done a substantial amount of restoration work to the building.

"During the period of government offices they changed and amended a lot of the rooms so we have tried to bring them back to what they looked like at the turn of the century when the astronomer's family lived here," Mr Bridge said.

"It's magnificent working in a heritage building of this nature. I have worked in modern offices and working in a lovely building like this has warmth of feeling and is great to work in."

The view from the Observatory house tower looks directly south down Fraser Avenue into Kings Park , 1899 and 2015. ( 1899 image: State Library of WA, 2015 image: 720 ABC Perth: Emma Wynne )

One of the best views at the house is from the top of the tower, which is not open to the public.

The tower roof, which was used to measure wind speeds, has 360-degree views of the city, stretching through Kings Park, overlooking the Premier's offices and as far as Subiaco oval, affording a unique view of the changes in the city landscape over time.