You can solve this by using the Rules module -- https://www.drupal.org/project/rules
Add a new rule: admin/config/workflow/rules/reaction/add
Enter a title and optionally, a tag
Select "After moderation transition" in the Node category for the "React on event" field
Click Save
Add a condition
for the "Select the condition to add" field, select "Content's current moderation state" in the node category
Leave the default "node" for the "Data Selector" field in the Content fieldset
Select "Needs Review" for the Value field in the Workbench Moderation State fieldset
Click Save
Add an action
For the "Select the action to add" field select "Send an email" in the System category
Next page, the TO fieldset
Click the "switch to data selection" button
Enter "node:author:mail" into the Data Selector field
Enter values in the Value field for the Subject, Message, and optionally the From and Language fieldsets
Click save.
You'll have a rule that sends an email to the author every time a node transitions to the Needs Review moderation state.