Chief executive Robert Ryan said IMB had acquired $16 million of the 2.99 million ordinary shares on issue.

The buyback was oversubscribed and the most successful to date.

The price for the shares bought back by IMB was $5.35 and after three rounds the Illawarra-founded building society has bought back 8 million shares, representing 20 per cent of the ordinary shares on issue.

Mr Ryan said the buyback was being conducted because IMB's board believed a mutual structure provided the best opportunity to deliver IMB's strategic goals.

The intention is to buy back all of the ordinary shares on issue over the next five to 10 years.

"The board is pleased with the progress of the share buyback program and remains committed to undertaking further buybacks over the medium term and implementing initiatives to support IMB remaining a mutual organisation," he said.

Mr Ryan believed the benefits for shareholders were liquidity and the ability to sell their shares for a very good price.

"There was no discount and they were at a very high market price," he said.

"And then on top of that they got the franking credits that are attached. The actual amount is considered to be part capital and part dividend. So depending on your own tax circumstances that is a real plus."

The next buyback is likely to occur at a similar time in 2016.

Wollongong stockbroker Doug Symes, of Novus Capital, encouraged shareholders to seek advice on the offer made by IMB.

Mr Symes said from a tax perspective it was not necessarily right for everyone.

"That is why people need to get consultation with someone who knows what they are talking about so they can assess just what is right for them," he said.

Mr Symes said some people had held their shares for a long time and may not understand the discount factor and that even at a discount the actual figure they end up with could actually be much better than they think they might get on the market.

It is the after-tax consideration that is the important issue.

"A lot of people don't understand how it all works together for their personal tax situation," he said.

Mr Ryan said there was no discount with the latest buyback which was probably one of the reasons it was oversubscribed.

"This time we went with no discount just to get the message out there that you should have a look at this event," he said.

Mr Ryan said coming into the IMB's 135th year the board was wedded to the mutuality and the membership and subsequent benefits of that.

"So they are over time going to buy back all the shares and take that shareholding out and make it just a member-owned entity again like it used to be," he said.

"The key thing for us is they are going to turn the company that way. So they are indicating that to the shareholders.

"So we are providing the opportunities to the shareholders to maximise their return now."