Progress on moving Auckland's historic Birdcage pub has been slower than expected, and relocation will now take until at least midday tomorrow.

Design engineer Adam Thornton said the nineteenth century pub was initially expected to be relocated before the day was through.

All 740 tonnes of the building are being moved 40 metres in a painstaking process to make way for the Victoria Park tunnel project.

Once the tunnel is complete, the entire structure will be moved back to rest on top of the tunnel.

The Birdcage, also known as the Rob Roy Hotel, was built 124 years ago and has been designated a heritage building.

It is being moved on a specially designed concrete track with a greased teflon surface at a cost of $2.5 million.

The Teflon-coated concrete beams the pub was sliding on were crossing moderately compounded ground, so it was possible they could sink a bit, said Mr Thornton today.

For this reason, the building's levels had to be monitored constantly and hydraulic jacks had been fitted which could lift the building if necessary.

"It only needs to drop about 10 millimetres for it to crack," Mr Thornton said.

"But it's all going to plan in terms keeping it nice and level, even though it's taking a little longer than expected. But it doesn't matter - we'll take as much time as is necessary."

New Zealand Transport Agency (NZTA) State Highway manager Tommy Parker told reporters that today was a "unique challenge" for NZTA staff and they were excited.

Mr Parker said the roof of the tunnel had been strengthened to take the weight of the Rob Roy but the weight of the fill used to cover other parts of the tunnel was heavier.

"It's a piece of history and we're really happy to keep it in its original position," Mr Parker said.

The Rob Roy

The Hotel and bar was built in 1885-1886 in Auckland's Freeman's Bay when Victoria Park was a busy industrial area that included ship builders, sawmills, a brass and iron foundry and glass works.

The Hotel was built by Samuel Jagger who employed architects Edward Mahoney & Sons, who designed many of Auckland's early hotels as well as St Patrick's Cathedral.

The Rob Roy had three storeys. The basement housed the kitchens, pantry, beer and wine cellars and servant's diningroom. The ground level had a bar, three sitting rooms and a serving room. Upstairs were nine bedrooms and another sitting room.

After Mr Jagger's death, the hotel was owned by Hancock and Co until it was sold to Tony White and John Banks in the early 1980s. It was later renamed the Birdcage Tavern. In 2002 the NZTA bought the Rob Roy/ Birdcage to extend the motorway.