The NZX said it would looking to the sharp gain in CSM's share price just as the listed shell announced its reverse listing plans, which involved buying The Good Brand Company and Me Today.
The stock went from $0.001 to $0.02 in early December and to $0.023 just before the deal was announced. The stock was placed in a trading halt on December 11.
"As part of NZX's routine surveillance practices, NZX Regulation will be assessing the trading and share transfers in CSM ahead of CSM's announcement on 11 December 2019 regarding the acquisition of The Good Brand Company Limited and Me Today," the NZX said in an emailed response to a question from the Herald.
The NZX said on Tuesday that it had suspended the quotation of shares in CSM - which is part-owned by Sistema founder Brendan Lindsay - pending more information about its backdoor listing plans.
CSM said last week it had reached an agreement to buy The Good Brand Co, a marketing company, and Me Today, which is backed by Grant Baker and Stephen Sinclair, who founded vodka company 42 Below with Geoff Ross.
The acquisition constitutes a reverse listing which, in accordance with NZX guidance, triggers a suspension of quotation.
The exchange said the process was designed to ensure that the shares can only be traded where the market is fully informed about all material aspects of a potential acquisition in the context of a reverse listing transaction.
"The suspension will remain in place until information contemplated by a notice of meeting or profile is disclosed to the market," the NZX said in this week's announcement.
The transaction values both the Me Today and Good Brand Company businesses at $5.55 million, including $1m of cash. CSM is valued at $2.1m.
Lindsay, who sold plastics company Sistema for $660m in 2016, last month paid $250,000 for a 12 per cent stake in CSM Group.
CSM was listed on the now-defunct junior board, the NZAX, before moving to the exchange's main board in July.
In its last annual report for the year to June, CSM - a former scrap metal company - announced a $1.3m loss.
The CSM transaction is the first to be undertaken since the NZX introduced new rules covering reverse listings in January this year.