After being dragged into a political stoush, West Australian Under Treasurer Tim Marney says the office of Treasurer Troy Buswell is partly to blame for a misunderstanding surrounding a financial briefing with Labor.
Labor last week accused the Liberals of espionage, saying they planted a stooge in the opposition's recent briefing with Treasury ahead of the March 9 election.
An official from Mr Buswell's office attended the meeting, but was not introduced to Labor MPs.
Under caretaker conventions, Labor had to submit a request to Mr Buswell's office for the briefing, which it agreed to, Mr Marney said.
The official was a Department of Treasury employee, on secondment to Mr Buswell's office, and having the staffer present at the meeting was a condition the office had placed on the briefing.
"I had assumed that they informed the opposition that they had agreed to a briefing request - given that the request came to the Treasurer's office - and that they informed them that that was a condition of their approval for the briefing to go ahead," Mr Marney told AAP.
"The Treasurer's office didn't get back to them and say `but we will have an office representative there'.
"No one told me that hadn't been said, so when the meeting started, everyone individually introduced themselves to each other - there wasn't a sit-down-around-the-table introduction process - so I assumed in that process that it had been made clear.
"It obviously hadn't."
Mr Marney said he deeply regretted not making it clear the official was present.
"But it should have been done by the Treasurer's office when they agreed to the briefing," he said.
Mr Marney also confirmed the Department of Treasury had not provided costings for Labor's Metronet rail project, but it had done so for similar projects, sourcing information from the Public Transport Authority and providing that through the Treasurer's office.
"But we didn't verify or validate any of those costs. In the time available and the detail we had available, we weren't able to validate the costs," Mr Marney said.
But Mr Buswell instead urged Labor to get its own costing from Treasury on the project, which forms the key plank of the opposition's election campaign.
Labor says it will cost $3.8 billion, but the Liberals put it at $6.4 billion.
And on Wednesday, Mr Buswell said the Liberals had discovered "yet another black hole" in Labor's Metronet budget of about $1 billion, saying it had failed to factor in additional rail infrastructure that would be needed to avoid bottlenecks between Bayswater and the CBD.