RSL veterans are disgusted by the theft of a World War I-era German mortar cannon from a new memorial garden in the South Australian town of Murray Bridge, which was due to be unveiled at Anzac Day commemorations.

Sub-branch president Ray Duthie said if the theft of the 400 kilogram-plus cannon on Monday night was a prank, it was not appreciated.

"This is a memorial and anyone who desecrates a memorial is the lowest of the low," Mr Duthie said.

"They are dishonouring those who made the supreme sacrifice for our country and those that came after them.

"The members of the RSL are extremely upset."

Club members and tradesmen were hard at work yesterday preparing the mortar site and surrounding memorial tablets for Anzac Day celebrations after more than a year's worth of fundraising and building to prepare the new site.

 "The mortar was the centrepiece for two (memorial) tablets and now they've put us back … it was going to be ready for Thursday," Mr Duthie said.

 The thieves appear to have had a hard time taking the relic, which club members believe to be too heavy for one person to carry.

It seems the weapon was been rolled down the hill it sat upon, making a mess of a memorial garden at the club entrance.

 However, members do not believe the green-coloured mortar would have suffered any damage during the heist.

 "I don't think so, it's a pretty sturdy piece of hardware," one member said.

 Although the weapon was deadly in its heyday, Mr Duthie said it had been thoroughly disabled and he was confident the thieves would not be able to make it operational again or find the ammunition required to make it dangerous.

Mr Duthie said, while insurance would likely cover the cost of the weapon - which he said was almost impossible to accurately value - the club did not believe it would be able to find a replacement if the gun was not recovered. 

The Murray Bridge RSL has been the custodian of the mortar since the 1950s.

A spokesperson for South Australia Police said anyone with information on the theft should contact police.

 "The cannon was stolen sometime between 9.20pm on April 22 and 8.45am on April 23 from the Princes Highway premises," they said. 

"It is of significant historical and monetary value, with an estimated value of over $40,000.

"RSL staff are keen to see it returned safely."

- Details: Anyone with information which may assist police should contact Crime Stoppers on 1800 333 000.