Like threading a needle, workmen carried out one of the most challenging jobs in the contract to strengthen the Royal Wanganui Opera House yesterday.

The historic building is the first of the city's public buildings to be brought up to earthquake strength and one of the keystones of the upgrade involved getting three 1.5-tonne steel beams into the Opera House.

The beams will create a new frame around the arch above the Opera House stage and will become a cornerstone for much of the earthquake strengthening.

Rick Grobecker, Wanganui District Council engineer and project manager, said the delicate work done yesterday was a "mini milestone".

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The job involved cutting two holes in the Opera House roof, before two 14m upright beams were lowered inside using two cranes.

Putting the 13m cross member in place was trickier because it had to be threaded horizontally between the holes in the roof, again attached to the second crane and then lowered to sit on top of the uprights.

Mr Grobecker said a new concrete foundation beam beneath the edge of the stage had been built to provide the base of the steelwork.

Once the new framing is in place the original cast iron work that decorated the arch will be put back in place.

Yesterday's lift hasn't been the only tricky job on site for main contractors, Wanganui firm WW Construction. Preparing for the foundation work has meant excavating 120 tonne of material but because the workmen are working within the confines of the existing building, it has all been removed by hand and wheelbarrows.

The seismic upgrade contract of $965,984 was awarded to WW Construction in April 2015 and eight of its staff began work a month later.

Mr Grobecker said the 16 cast iron internal columns have been removed which means the dress circle is currently supported by a scaffold system, which is also being used to access the ceiling space.

Workers are covering the ceiling in a layer of structural plaster which itself is tied back to the ceiling supports by 3870 stainless steel ties.

Other local firms in the contract include BPL, which handled the structural component of the upgrade, while structural steel fabrication work is being undertaken by Elite Mechanical, with plaster being installed by Allan Tong Ltd.