An explosive text message from the chief of NSW Police to his deputy about equipment issues in the critical final hours of the Lindt Cafe siege has been revealed.

Details of the text, sent from Commissioner Andrew Scipione to his deputy Cath Burn, were brought to light in a shock development on Tuesday during the inquest into the deadly December 2014 stand-off.

The text, which Ms Burn said she received and converted to an email that she sent to herself at 10.37pm on the night of the siege, centred on improving police equipment.

“Catherine, I have had a quick chance to talk to the SSG (Specialist Services Group) team that were forward tonight and it has become apparent that we should be preparing a fresh bid for new equipment that is necessary,” the text, read aloud by Ms Burn, said.

“Can you please make sure we get some advice from Mal as to new electronic images / audio / intelligence equipment that we may need for the future.

“I will need this soon in order to make sure we are appropriately prepared for the future. I’m happy to discuss in the morning. Thanks again for your hard work today, see you bright and early in the morning. Regards, Andrew.”

Ms Burn had earlier clocked off at around 10pm at the direction of Mr Scipione, the inquest has heard.

The email was not included in a bundle of documents Ms Burn tendered to the inquest some weeks ago after she and police IT staff swept her records.

She said somebody from the police professional standards branch had gone back and checked the emails she’d produced to the inquiry and realised it was missing.

NSW Police Commissioner Andrew Scipione is set to face the siege inquest this week. Photo: AAP

The document was identified on Monday night and produced at the inquest on Tuesday morning.

“It was a bit of a shock,” Ms Burn said.

Burn did not think she needed to keep texts

On Monday she was forced to explain why she deleted text messages sent during and soon after the siege.

A telecommunications provider has spent the past fortnight trying in vain to retrieve messages Ms Burn sent throughout the 17-hour December 2014 stand-off.

Ms Burn told the inquest she did not believe the texts needed to be kept, and deleted them as a matter of course.

The deputy commissioner said many of the texts related to setting up briefings or meetings, while others were from members of the public offering police support.

She said other than texting another senior officer soon after the siege ended, she couldn’t remember any that pertained to police storming the building.

The inquest was also told it was “highly unlikely” any of the unrecovered texts related to progress in negotiations.

“To my knowledge and my recollection none of the texts I had were texts I thought I needed to keep, that I needed to retain,” Ms Burn told the inquest.

Ms Burn insisted on Tuesday that she was not made aware of equipment issues while she was on duty during the stand-off.

“There was nothing brought to my attention about a concern about lack of equipment,” she said.

Nor was she informed about possible issues with resources available to the police negotiations team.

“Nobody brought that to my attention,” she said.

The inquest continues.

-AAP